In order to fully understand why it was so hard for Mike to express how deeply he loves El, and why his speech at the end of season 4 was one of the biggest, most important moments for his entire character, we need to look at not just who Mike is as a person, but also everything that has happened since he met her.
Every single time he opens up his heart to her, something horrible happens to her or she's taken away from him almost immediately afterwards.
1x08; he's an awkward little ball of feelings that are way too big for a boy so young. He makes a nervous attempt at confessing and asking her out on a date; when he can't find words that she'll understand, swoops in for a kiss instead. She lights up immediately and smiles. It's a brief moment of hope and pure happiness. Maybe they can have some semblance of a normal life and be normal kids after this is all over.
Minutes later, all hell breaks loose-- they're almost shot, El pushes herself too far until she can barely move, she's almost taken away by the Bad Men, the Demogorgon appears, and she uses the very last of her strength to sacrifice herself to save him and their friends.
He has to watch helplessly as she disappears.
He spends a year caught between believing she's dead and hoping she's still out there somewhere (but if she is alive then why won't she talk to him anymore...?). Kept silent under threat by the lab, he can't confide in anyone or even acknowledge her existence, not with anyone except those involved... but everyone else is keen on moving on and pretending it never happened. He can find some solidarity in Will, at least, who is in a similar kind of emotional turmoil... but it's not the same and it's not enough.
2x9; he is finally reunited with El, and she runs into his arms like she missed him too. She tells him that all those nights he called out to her, she heard him; she was there reciprocating his feelings the whole time.
In a burst of emotions that he's been forced to suppress for an entire year, he lashes out at the reason they've been kept apart (Hopper), screaming and sobbing. It's a massive catharsis for him, and for once an adult is understanding enough to hold him and not punish him for it.
Minutes later, she is going to go headfirst into a pit of monsters, the place where Mike had just firsthand witnessed dozens of people (if not more) get ripped to shreds only hours earlier, and she is going to attempt to close the Gate-- a feat that he knows may take every ounce of her power, just like last time. He cries. He can't lose her again. She promises he won't, and before she can seal that promise with a kiss, they're pulled apart again.
He has to watch helplessly as she drives away.
3x1; all seems to be okay now. He and El are happily together, and he feels comfortable enough to be playful, romantic, and intimate with her. It's the most emotionally open we've ever seen Mike thus far.
For reasons he can't understand (bc there's no way Hopper explained himself beyond "I'm in charge so do as I say or else"), Hopper is angry about it and threatens to never allow him to see her again: the one thing he fears most.
He panics big time and fucks it up in the process by lying to her. During a frantic attempt to apologize while also abiding by Hopper's rules, he runs into her at the mall. He panics again-- if anyone finds her here, and knows that he was here too, it's all over, and Hopper surely won't hear reason. El dumps him cold on the spot, spurred on by Max and her rebellious attitude (and without any context of course). He isn't given much opportunity to respond. He knows he's in the wrong for lying to her, so what could he even say...?
He has to watch helplessly as she drives away.
It's a hard blow, and he retreats back into himself, unable to get any joy out of playing D&D (which he clearly hasn't lost interest in), back to the deadpan sarcasm and accidentally snapping a little too harshly at anyone whom he feels would take El's place.
3x6; no one seems to understand the danger El is putting herself in. Everyone is berating him for worrying about her safety. He's seen firsthand what these monsters do to people, he's seen firsthand how El pushes her abilities too far. No one is listening.
The words "I love her and I can't lose her again" burst out in his desperation, perhaps before he's even had a chance to realize how deep those feelings run, despite whatever protective walls he's tried to build around his heart to keep it from getting broken again.
Soon after, all hell breaks loose. El is nearly killed several times over, her leg is ripped open, she pushes herself so hard that she breaks herself and loses her powers completely. Her father is taken from her. She's shattered by all of this, and there's absolutely nothing he could do or say to make it better.
She tells him that when he admitted he loves her, she heard him, and indeed she loves him, too... But now she's leaving.
He has to watch helplessly as she drives away.
4x1; they've been apart for a few months, and write letters back and forth to each other. El's letters paint a picture of an ideal new life: she and the Byers family are doing well; she's starting school and it's going well; she's made new friends, she likes her new home, everything is going well. She seems to be thriving. She sounds happy, maybe even happier than she had been living in Hawkins. Maybe Max was right, maybe she's better off being her own person without him, and maybe the respectful thing to do is step back... It's a small insecurity that creeps up subconsciously. In his replies he holds back, afraid of clinging too hard.
Though there's little logic in it, he's afraid that if he tells her he loves her again, another disaster might strike and this lovely happy life she's finally found might get taken from her. After all, that's what always seems to happen when he does.
4x2; after months of waiting, they can finally see each other again. He wears her favorite colors, picks a handful of flowers for her, and falls happily back into step with how they used to be. Soon that same day, however, reality becomes clear and the facade crumbles. People he was told were her friends show up to torment and publicly humiliate her. She had been lying. She isn't happy here, she hasn't healed, she is right at the edge of a breaking point that he doesn't see coming at all. He can't believe she would lie to him, she's not the kind of person to lie... especially not about something like bullying, something that she was always so understanding about with him.
On that logicless subconscious level, he wonders if it's all his fault-- he should have known somehow, he should have been there for her. She protected him from his bullies, he should have protected her from hers. He tries to come to her rescue. She runs away from him.
He's helpless to save her, again.
4x3; after a night to process everything that happened-- and deciding that the betrayal he feels from her lying to him is nothing compared to the turmoil she must be going through right now-- Mike approaches her in the gentlest way possible, wanting to listen and trying to understand. El, however, isn't receptive at all to his attempts at reassurance. She is at an all-time low, she's given up. She believes she is unlovable, irredeemable, a monster, just a thing that doesn't even have those superhuman abilities to compensate anymore. Mike can't believe what he's hearing-- doesn't she know that she's always been so much more than her powers? She's always been so much more than what she lacks in quote-unquote "normalcy"... None of those things matter, they have absolutely no bearing on whether she's worthy of being loved, because he loves her, completely regardless of any of these things. He always has...
El starts flinging his restrained words back at him, the products of his insecurity and trauma-induced fear. That fear takes hold yet again, and he stumbles, afraid of saying too much or not enough, because surely both could result in pushing her away-- she's retreating, hearing none of it; nothing he tries to say consoles her.
Moments later, local police come knocking. She's taken away in cuffs, and she's so broken inside that she won't even look at him when he chases the police car down the street and promises he'll get her out somehow...
Once again, he has to watch helplessly as she drives away.
4x8/4x9; after days of driving through the heat and dryness of southwest desert, having narrowly escaped being shot at with military-grade assault rifles, witnessing the death of and burying a man whose last words were that El is in danger... After watching dozens of people get mowed down by a sniper in a helicopter, and watching that same helicopter be smashed into the ground in a ball of flames...
There she is. Just as powerful and beautiful and alive as she's ever been. When he runs to her and embraces her, she looks at him like she can't believe he's real. She's beaming a smile right from her soul and it's like all the insecurity and self-doubt that have plagued them both just vanish from existence now that they're in each other's arms again.
Like always, however, the universe comes crashing down soon after. Max is marked for a gruesome death and all of Hawkins is in danger. They're miles away and helpless, and the only possible way for El to save everyone is if she goes in alone. She's stronger than ever, but so is her foe. Once again, she descends to face all the demons of hell on her own, and Mike can't do anything.
She's losing. She's choking. She's dying. He's helpless.
He must be cursed. He must be. Being with her, loving her, allowing himself to admit he loves her, it always brings only pain and suffering and loss. His heart is so full that it's aching, it's bursting out of his chest, and he can't contain it any longer.
She's going to die and it's going to be all his fault, because he fell in love, and it's cursed her.
Just before it all crumbles into utter despair, the earnest support from his oldest and dearest friend-- one who's always shared and understood his feelings of helplessness-- sparks a light of hope in him: "You're the Heart." You're not helpless. You can save her.
The words that come spilling out of Mike's mouth are truer than any he's ever dared to speak before, and it's the most terrified he's ever been, but he has enough courage for this moment. Despite all of the fears that have been building, stifling, choking him to death for years-- fears that the light of his life will inevitably disappear again, and there's nothing he can do to stop it-- despite it all, he pours out his heart to her.
He loves her. He's always loved her. He loves everything she ever was, is, and could be. He can't imagine a world without her in it. She saved him, in every way a person can be saved. And he needs her to live. He believes in her.
And it works. It's music to her ears.
