Through what we've read so far in the book, we can tell that Lennie is very dependent on George, for several reasons. One of the things that really causes this is because he is so forgetful. We can tell through a couple of examples that he forgets things very easily. He even forgot that his aunt was the one who would give him pet rats. He forgot his own aunt's name! Other things like how he forgot how he can hurt rats is are more examples that he forgets these things very easily.
Another reason that he depends so much on George is that he is overall just mentally challenged. He wouldn't think that much about he kept killing the rats that he wanted to pet so much. He would just keep killing them, and not thinking twice about it.
Lennie also depends so much on George because he really trys to model himself after George. In several instances, the author talks about how Lennie would copy what George was doing, and then look up to check that he was doing it right. Lennie might be in a much worse situation than he is in now if he didn't have George to shape him up.
I think that George knows this. That's why he doesn't just ditch Lennie for a better life, or in his words "get a girl". If he didn't look after Lennie, something bad might happen to him. George seems like a really good guy, and he really just wants to look after Lennie.
He knows that Lennie can do some bad stuff if no one is there to look after him. The author hasn't gone through it too much, but he talked about how they were both chased out of town. For what, we don't know yet. But I think when we find out, we'll have even more insight into why George looks after Lennie.
I think that he really took a parental position to Lennie, liking a nagging mom or dad. He really cares about him for some reason. Like the way he took the rat away from him showed that he just wanted to help. The rat was going to bite him, and then he was going to be sad that it was dead.
There might also have been an event that happened to them that brought them together. We'll just have to read the book and see what happens.
