1) I believe that Lennie is dependant on George because he can't do much on his own and he can't really think normally. Also because he forgets a lot of things very quickly. This is also why he can't hold a job which makes him dependant on George. Lennie is also very gullible and child like also making him incapable of holding a job.
2)I think that George keeps Lennie around even though he could have a better life without him because he cares for him and possibly because he knows that Lennie would've done the same thing for him if their lives were switched. Another possibility is that George might owe Lennie something and this is his way of repaying him.
