George and Lennie have a special bond that, unfortunately , is viewed with suspicion by others.
"Ain't many guys travel around together … I don't know why. Maybe ever'body in the whole damn world is scared of each other"
Lennie's poor skills and his overwhelming panic ultimately cause the death of Curley's wife and still George feels responsible for him. Lennie, mentally challenged and not aware of his own strength; also having a need to touch everything, has already killed, amongst other small animals, a puppy by being too rough. This foreshadows events to follow.
George took responsibility for Lennie a long time ago, after a promise to Aunt Clara, as Lennie is compromised in social situations and does not know how to behave. His actions are often misintepreted and George, as Lennie's caretaker, has had to get Lennie out of difficult situations before.
Lennie is childlike and often described in association with animals. As with animals, he is unpredictable and completely unaware that his strength can cause harm.
George takes his responsibility for Lennie very seriously and without him, George
is friendless and alone.
The friendship keeps the dream alive of having their own farm and Lennie is instrumental in keeping that dream going throughout the story. George needs this relationship as much as Lennie depends on it.
It does sometimes consume George and his anger is evident - "I been mean, ain't I?" - but he makes, what he sees as the ultimate sacrifice, when he feels that the only way to protect Lennie from himself ,as well as from the Law, is to kill him.