How do Curley's wife and Crooks symbolise loneliness?I know they are both lonely people but I don't know how they symbolise it

The whole novel Of Mice and Men is filled with lonely characters.  Candy is lonely as an old man with no family and no secure future.  George is lonely for companionship because he takes care of Lennie and because he is a migrant worker and never has the time to cement friendships or relationships.  Curley and his wife are alienated from each other, and Curley from the whole ranch it would seem.  Crooks is physically lonely, as no one socializes with him, and there is no one else from his race or culture to socialize with.

Curley's wife and Crooks do not, by themselves, then, symbolize loneliness.  Rather, the whole novel portrays the all-encompassing loneliness of a broken society in the depths of the Depression, and of those who are most vulnerable in it.  This includes Curley's wife and Crooks, but it also includes almost everyone else in the story.