According to the crooks, Lennie will go to a mental hospital if George goes away. However, as far as that goes, that’s because Lennie is written in this book in such a way that it is obvious something is wrong with him. The reader never knows what, specifically, since Steinbeck doesn’t tell us. Regardless, at this point in the book, Lennie simply can’t take care of himself.
George is the one that keeps him from going off the leash. That’s all there is to it.
Now, I haven’t read “Of Mice and Men”, but I know enough about the characters to answer this question. I’m probably leaving out some major details, but I don’t have time to read this book right now.