Crooks is the only farmhand who is African American. Living as a black man in the Depression Era, and among a bunch of rogue men of their generation, rendered Crooks as an outsider to the group. The treatment that Crooks got was purely segregationist. He had to live in isolation, move his things away from everyone else, and people would not really speak with him, or to him.
Isolation in an already-barren and dry place would make anyone grow quite depressed. Surely there is no exception with Crooks, who aged into a grumpy, lonely, bitter, and cynical man.
All this being said, Crooks had more than enough reasons to be cynical. First, he has absolutely no other support systems in the farm. Curley does not care about him and "gives him h*ll", as George finds out. The other farmhands, despite of thinking Crooks as a hard worker, and an "OK" kind of guy, still dub him as a "n***er" and find that to be a controversial matter to speak of. These are the first descriptors that George gets to learn about Crooks...before he ever got to meet him!
Also, living in isolation, Crooks has plenty of time to think about his life, or lack thereof. Like any other man, he needs to make a human connection, reach out to others, and find a source of support in case something happens to him. All that we know is that a horse accident rendered him crooked. Having a physical condition, combined with living (literally) fending for himself, cannot be a reason to cause him to smile every morning.
However, we know that Crooks changes his tone after he hears that Candy, George, and Lennie had a plan for themselves. They want to leave the farm, start their own, and live together. While Crooks finds that as a prime reason to spill his venom and downgrade the men's dream, he hears that Candy can actually make it happen; there is money to put down, after all.
To this, Crooks shows his true self: the man who really hungers for humanity, and to be treated humanely. This is the reason why he even offers his own services for free, denoting that his cynical and bitter life is nothing but a front to cover deeper and more painful emotions.