Both Lennie and Crooks live isolated existences on the fringes of society -- Lenny because of his mental disabilities and Crooks because of his race. They also both represent a section of society that seems to have little chance of upward mobility. They work on the farms and in the factories but don't and probably never will own any part of them.
Despite their low station in life, both Lennie and Crooks seem to carry a kind of dignity with them. This is a trait Steinbeck was masterful at portraying -- dignity in the face of the harshness of life.
Both Lennie and Crooks attract attention because of their physical appearance (Crooks being black and disabled, Lennie being large). Their appearance gets them into trouble - Lennie's size with Curley; Crooks' colour with everyone except Lennie. Both men are subject to the prejudices of the time: racism and a misunderstanding of mental challenges. They are both viewed with suspicion and resentment and are subject to humiliation. However hard either man tried to overcome their challenges, they would not be able to escape their position.
They were all simply outcasts in their society. They were all lonely in this whole world and wanted a better life for people to be caring about them, so they have to now find good things in life to supplement their loneliness, making us readers sympathies with them, perhaps they want a sense of belonging and to enjoy simple pleasures and have their rights. They were already known to be doomed for disaster and the readers had to watch in utter helplessness in the events that unfold that they have already known