What do Crooks, Candy, and Lennie all have in common that allows them to bond briefly in Crooks' room?
All three of the men have a dream to own their own place, to belong somewhere, and to have someone to care about them. During the Great Depression, many men had to move around to find jobs. Most of them did so alone, and this is why George and Lennie are somewhat of an oddity. They care for and need each other, and this is what Candy and Crooks also want. Candy knows he's not going to be able to work much longer before he's told he's of no use anymore. He desperately needs somewhere to go and to have a sense of belonging. This is also a time of racism, so the feeling of loneliness is even worse for Crooks. He's even separated from the rest of the men and not allowed to go into the bunk house. Even Curley's wife expresses her own loneliness and her need to belong.
They are all social outcasts and suffer the isolation due to a lack of true companionship.
Crooks is an outcast by both his race and his physical deformity of a crooked spine. He covers his loneliness with books, yet realizes he is lonesome. Candy is also physically disabled and feels isolated. His quick response to join loners George and Lennie in their dream is an indication of his lonesome state. Lennie is mentally handicapped and George is his only companion. Yet, even George cannot communicate with him as an equal.
These men, gathered together in Crook's room, share a common issue of isolation, as well as the inability to fit in anywhere.
As Curley's wife implies, these three men are social outcasts. They remain behind when the other ranch hands travel into town for drink and women. Candy, Crooks, and Lennie are all crippled: Candy lost a hand in an accident with some machinery, Crooks has an injured back, and Lennie is "slow" mentally. These physical and/or mental challenges have caused them to be placed on the periphery of the social order. They do not easily fit in with the others and are able to bond, albeit for only a brief time, when they are all in Crooks' room in Section 4.