Mike didn't love the El "the moment" he saw her. But there IS someone he did...
It wasn't romantic love, as Mike and Will were too young. But it was more than just a playground friendship. Most kids make friends simply playing together and probably don't remember the day they met very well.
But Mike remembers it vividly, and so does Will. And the way Mike tells it in s2e8, he had feelings for Will when he saw him on the swings alone:
Mike loved Will when he saw him. He didn't want him to be lonely. Mike also felt alone. Perhaps he sensed they were different from the other boys somehow?
He could have just joined him on the swings or said they should play. Instead he asked if they could be friends. The way Mike tells it, it had the intensity of a proposal.
"And you said yes... you said yes..."
"It was the best thing I've ever done."
It took Mike longer to feel love for El. But for Will he loved him at first sight. And in more than a friends way, but in a BEST FRIENDS way, the way kids can and do love each other. The only thing different is that Mike and Will continued to feel that same intense bond they had as small children, when they were older in s2. And we now see it maturing into something else in s4 (and s5!)...
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Mike's trying to be a different person while he's with El and even feels like he has to hide his nerdy self from her while he feels like he can fully be himself with Will and Willl makes him feel loved instead of insicure and Mike wants to feel needed and Will needs him and understands him and El doesn't
me when mike and his friends including steve jonathan nancy etc gather around to talk about protecting will from vecna and mike screams at someone about will's safety and everyone goes silent