nane's words

I think my view of the Wheeler family's middle child has always been a little deeper because I can see myself in him. To begin with, Eleven and Will fall in love with Mike because he gave them the bare minimum, something they had never received before. And when Mike notices their affection, he returns it—because we all like receiving affection—and, as a result, he unintentionally makes them place expectations on him.

The problem with Mike Wheeler is that he doesn't know how to be the person other people imagine him to be because he's a complete idiot. Those people eventually realize what an asshole he really is and start suffering because of him, and Mike doesn't really know how to react to that because he simply can't change who he is.

I believe the ending they gave Mike represents the idea that either those people (Will) eventually give up on him and move on with their lives, or they end up losing themselves before they can do that (Eleven).