What do Crooks, Candy, and Lennie all have in common that allows them to bond briefly in Crooks' room?

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.