Why do George and Lennie stay together?

Lennie stays with George because he has no one else. George cares for him, watches over him, and does his best to protect him. Without George, Lennie would likely have been put in a special home.

George stays with Lennie for a few different reasons. First, he promised Lennie's Aunt Clara he'd look out for him. Second, George likes Lennie.... he also likes the company, even if he often finds him frustrating.