I was thinking that Mike doesn’t really know how to show his affection for Eleven once she gets back. I mean, obviously, he has deep affection (and love) for this girl but he’s twelve to thirteen so he doesn’t really know how to express that at first. He wants to be close to her all the time but his dilemma is how he’s supposed to do that without grossing her out, or worse, scaring her? So he stays nearby but not too close and always gets a thrill when Eleven sits next to him and leans into him, resting her head on his shoulder. Only then does he quietly grab her hand and rest his head on top of hers.
And when they’re walking and he’ll hold her hand, she’ll intertwine her fingers with his and he’ll gently pull her arm close to his side, which leads to the added bonus of pulling her close too.
Sometimes, though, he’ll kiss her on the cheek. Kissing her on the lips always feels like such a big thing to do, although honestly (and the thought always makes him blush so much he’s sure his face is going to melt off one of these days) he’d like to kiss her like that all the time.