George takes care of Lennie who is mentally handicapped, although physically very strong, a big bear of a man. George depends on Lennie for his companionship, even though George complains about having to take care of Lennie, he really needs him emotionally.
The two share a friendship that is more like a brothers relationship than that between strangers. Lennie, because of his immense strength and his lack of an adult's mental capacity to understand his own strength, is capable of killing with his bare hands. He doesn't mean to do this, but the simple, child-like Lennie can't resist petting soft things.
George looks after Lennie time and again when he gets into trouble. The two run away from jobs where Lennie has done damage with his super strength. George feels compelled to take care of Lennie, having promised his aunt that he would do so.
In the course of the story, we learn that Lennie and George share a secret dream of owning their own farm someday. This story, like a child's bedtime story, soothes Lennie, and George uses it to control Lennie's behavior to a certain extent. He constantly reminds him not to misbehave.
But unfortunately, Lennie does not spend every minute with George. He cannot control the man's behavior all the time. In the end, Lennie's actions lead to a tragic death. Sadly, George feels that the only way to take care of his giant friend, before others can punish him, is to kill him.