George is the character that looks out for Lennie. With out George, Lennie most likely would be lost. You see, Lennie's not exactly the brightest person on Earth and George keeps him in line, it's almost George's responsibility to be the care taker of Lennie. I think what keeps the two together is the bond that they've grown with each other and it wouldn't be fair of George to dump Lennie on his own, he would probably never make it.
2. Some acts that Lennie "accidentally" commits during the story is the killing of Curly's wife and killing one of the puppies. I guess you could say that Lennie's too strong for his own good, or mind. It's like he doesn't know his own strength. When Curly's wife told Lennie to stroke her hair and feel how smooth and soft it was he started pressing on her head a little too hard, when she asked him to stop he wouldn't and ended up strangling her to death. With the puppy, Lennie was simply petting it too hard for the little puppy to handle. .
3. George and Lennie share the dream of some day owning a ranch of their own with lots of bunnies. This dream controls Lennie in a sense, it's all he ever thinks about, his ranch and his bunnies. So, that as well as George, keeps Lennie in line. .
4. The tragic ending of the novel is simply the killing of Lennie. George had no choice but to put this troubled guy to his death. It was basically for his own good, truly. Lennie was causing wrong right and left and it was uncontrollable. So, George shot lennie with Carlson's gun. .