Since like two people wanted it, I made a shitty analysis thing on the fundamentals of Shouto loving Izuku, and thus part of why I'm a hopeless tdizu enjoyer. (Their chemistry aside, I just think they're neat.)
Also, I shouldn't have to say this but: I DID THIS FOR FUN. IF YOU DON'T LIKE IT...IDK! HAVE A GLASS OF WATER OR SOMETHING!
THIS ALSO CONTAINS MANGA SPOILERS. (I use pictures!)
READ AT YOUR OWN RISK.
(The combined pages are ordered from left to right even though it's read as normal. Sorry, I know that's confusing lol)
Most people agree that tdizu really takes off during their match at the Sports' Festival in Chapters 38 - 39, Todoroki vs Midoriya and Todoroki Shouto: Origin.
I agree. However, Shouto initially shows interest in Izuku during the USJ attack.
He has a first glimpse of Izuku using some of One for All's power (Izuku leaps out faster than anyone can blink), as well as seeing something going on between Izuku and All Might. It was enough that he decided to call Izuku out in a declaration of war before the Sports' Festival really began.
(It also doesn't help that Izuku and All Might are just really bad at hiding their relationship in general.)
And then later, during the calvary battle, Shouto felt compelled to use his flames because of the way Izuku was coming at him. It was only a little bit, but it was enough to rattle Shouto into calling Izuku out again (although this time, in secret).
That all leads to this conversation, where we learn that Shouto has been abused by Endeavor since birth because of a petty inferiority complex his father has concerning All Might.
Endeavor abused him to the point of destroying their family. He doesn't want to be anything like him and only wants to use his mother's half of his Quirk, but this pigeonholes him into a specific mindset: that he can never be someone good so long as his father's blood and Quirk is inside him.
But enter Midoriya Izuku, who doesn't know the meaning of the word "irredeemable".
He's also crazy. I say this lovingly and with no ill intent.
The famous, "It's your power, isn't it?!" and "Where are you looking?!" lines backed by Izuku's actions jog Shouto's memory, and he remembers words his mother said to him when he was young (before her mental breakdown and his subsequent facial scarring). It's about how he doesn't have to be bound by his father's blood and could be whatever kind of hero he wanted to be instead of the tool for power and recognition his father wanted.
Izuku mentions that he forgot himself during this fight, but he just became so frustrated by Shouto's situation (especially after having run into Endeavor right before the match) that he had to talk to him in any way he could.
He doesn't fight to win here. Rather, he fights to save, which is the essence of what being a hero means to Izuku. It's just the kind of person he is at his core.
He sees this sad, lonely person robbing themself of their agency, and he can't stand it. He can't just ignore it and continue on. He maims himself just to get his point across to Shouto, too. He goes out of his way to make Shouto see him and only him. For all intents and purposes, they may as well have been the only two people in that arena then, because that's all they saw at the very end.
It's this emotionally driven fight where Shouto really uses the fire half of his Quirk in battle for the first time, after previously resolving to reject that part of himself completely. He thought doing this meant he could reject everything about his father (who, while a respected hero, was not a good person). It's the reason why his first hero costume looks the way it does.
(While I do think it's an ugly costume overall, it was something born out of desperation to distance himself from an abusive parent, so I just feel sad looking back at it now lol)
Shouto still wanted to be a hero despite all this, but he'd lost his way and much of his identity as an individual. He only seemed to be seeking heroism out as a type of revenge against his father. Ultimately, he didn't think he could carry on into the future without rejecting parts of himself, and this mindset was leading him down a dark path.
But through his match with Izuku, he was reminded of why he wanted to become a hero in the first place, and he even states afterward that in that moment, he'd forgotten about his father entirely.
That fight shakes his entire worldview, and he's left with a lot of mixed feelings for the rest of the Sports' Festival. He goes back to not using his flames and becomes very unsure of both himself and his path leading forward. This causes him to later lose his match with Bakugo despite Izuku's plea for him to keep going.
His flames only flare at all because Izuku calls out to him, but they quickly die when Shouto's resolve weakens, and he loses his will to fight.
He changes after this, becoming more open to people and himself. He even goes on to visit his mother in the hospital for the first time to reconnect and apologize for not coming sooner. Eventually, he grows into the beloved AirCon (air conditioner) Hero that people only refer to as "Shouto" and not just "Endeavor's son".
And it all started because Izuku reached out to him.
It takes nearly the entire story for Shouto to come to terms with himself, but when he does, he makes a point to tell Izuku, "Yes, this is my power. Thank you."
(This is shown via flashback in Chapter 352, Ultimate Moves.)
The move Shouto has created here, Phosphor, is a unity of both halves of his Quirk. It's a cold fire, symbolic for his acceptance of himself and his path going forward.
Now, I'm just a simple fan, but the panel layout is very intentional here, and Shouto's heart lining up with Izuku's name? The one who jumpstarted his flames and led to him accepting himself?
Shouto spends the rest of the series (and I assume his whole life) looking Izuku's way with some of the softest, gooiest looks anyone could ever give a person. In official art/merchandise, some Horikoshi sketches, the manga itself, the anime, etc. He took the "where are you looking" thing very seriously, and he hasn't looked away since.
His fondness for Izuku is really just that obvious.
And I'm completely normal about this... (What if I explode, Horikoshi? What then?)
Anyway, all this to say, Shouto fell for Izuku because he's the one person who cared enough to tear down his walls and force him to really look at himself, so I hope I'm invited to the wedding because I give the best gifts overall.
Just saying.