Okay we already know that Mike is just an Intensely Emotional person. He feels things strongly and unapologetically and with his whole heart, and his absolute adoration of this weird and amazing girl is one of those things. Like, at first he just wanted to help her bc he's good and fair and honorable, and it was the right thing to do; he would have done the same no matter who he'd found wandering scared and alone in the woods. He could see how terrified she was, and while having psychic powers may have made her even cooler, it didn't change a thing about his resolve to protect her. But before long that drive to protect her turned into just a drive to be near her. He wasn't dumb or naive, he knew what a crush felt like, and by that last night at the school he knew that's what this feeling was. He knew he wanted to hold her hand and take her to the school dance and be with her every day.

Absence doesn't always make the heart grow fonder, but it sort of did for him. She saved his life and his friends' lives, flipped his entire world on its side, and then she just turned to dust. And in the aftermath of all that trauma, as he dealt poorly with his own stress and his own mental health, he just kept thinking that if he could only talk to her it would make everything better. She became every hope he had for his life feeling alright again.

He doesn't know the depth of it yet, but he does absolutely know that he's in love with her. She amazes him every single day, and just looking at her makes his heart feel funny and his face turn pink. TV shows have taught him well enough what that means, even if he can't quite understand it. He wants to be her boyfriend and do real boyfriend-girlfriend things, like go see movies and share milkshakes and kiss some more maybe and do that boring slow dancing thing teenagers do. He wants to give her a normal, real, happy life, like the ones she's always seeing in movies here. And as long as they're both in this world together, he intends to.