George and Lennie are ranch workers. Lennie has a disability, which is why George takes care of him. Lennie can do many stupid things that piss off George and makes him wish to live alone.
Ranch workers are considered the loneliest guys. They don't have a family or a home. All they do is work on a ranch until they get in jail and rot there. However, George and Lennie aren't like this. They have a future and goals they want to accomplish.
The book is set during the Great Depression. During this time, jobs were scarce. This can be seen in George, as he really wants to keep their new job as he knows it is really hard to find another one.