I just dont understand how Mike saying "I loved you right then and there" makes any sense. I am not even saying love at first sight cannot happen, it can irl sometimes and it is used in fiction. But for Mike abd Eleven, it doesnt make sense because we can see their love wasnt love at first sight. It completely goes against the established logic of the story.
Simply put, it's either a retcon or just plain bad storytelling.
There was no established "love at first sight" or anything of the sort. Mike was disappointed initially when he found El because he was trying to find Will. He had no initial interest in her until he thought she could help them find him. He wanted to keep her safe, yes, especially after learning the "bad men" were looking for her. But none of that changes the fact that his original plan was for her to be "sent back where she came from."
This isn't some knock on their relationship. It's simply a statement of fact. Everything that could be even remotely described as love developed after the fact. So, yes, "I knew right then and there that I loved you" is a load of bullshit. Either Mike was lying to try to make up for his previous inability to say it, or the Duffers are just disingenuous hacks who don't know their own story and characters well enough to build a story with continuity.
I know where I'd put my money.
Little wonder that so many people, myself included, thought this all to mean that Mike was just telling El what he thought she wanted to hear. The fact that he was intended to truly be in love with her from the beginning is some really screwed up storytelling, especially considering he met her while searching for Will and he said this right in front of him.
Nevermind all his weird behavior from the end of season 3 to the finale. It was only because he was insecure and thought he wasn't good enough for her, so, naturally, she would leave him someday.
Nevermind that all of their interactions when they're not in a dangerous, stressful situation are forced and awkward.
Nevermind that Mike treated Will like he should have treated El all along, while Will was also written to be in love with Mike.
Somehow, it makes far more sense for Mike to have fallen for El the moment he laid eyes on her and to have known her better than anyone than for Mike and Will to share a deep friendship that could have grown into something more.