Loneliness

George and Lennie aren't lonely because they have one another

"If you don' want me I can go off in the hills an' find a cave. I can go away any time."

"No—look! I was jus' foolin', Lennie. 'Cause I want you to stay with me."

Crooks, however, is lonely: he has to live alone:

"A man gets too lonely an' I tell ya' he gets sick"

It's typical for ranch hands to be alone:

"Ain't many guys travel around together," he mused. "I don't know why. Maybe ever'body in the whole damn world is scared of each other." 

Curley's Wife is lonely, as the only woman on the ranch with nobody to speak to:

"I never get to talk to nobody. I get awful lonely." "I cant talk to nobody but Curley. Else he gets mad"

Crooks is a victim of marginalisation:

"…You go on get outta my room. I ain’t wanted in the bunk house, and you ain’t wanted in my room." "Why ain’t you wanted?" Lennie asked. "’Cause I’m black…"

As is Lennie:

"If he finds out what a crazy ******* you are, we won't get no job"

Candy is as well, his 'uselessness' is represented though the shooting of his dog.

Another victim is Curley's wife, she is called a 'tramp', 'rattrap', and 'tart' frequently. She uses any upperhand she has to be unkind to other marginalised characters:

"I could get you strung up on tree so easy it ain't even funny."

Crooks too does this, using Lennie's innocence to torture him:

"Just s'pose George don't come back"

All the characters in Of Mice and Men have dreams. Though, the title of book does effectively tell us that these are unnattainable dreams:

The best-laid schemes o' mice an' men
Gang aft agley, (often go awry)
An' lea'e us nought but grief an' pain,
For promis'd joy!

Lennie dreams of "tending to the rabbits", and George too dreams of one day living "off the fatta the lan'" Candy joins them in this dream, which is his only escape from the loneliness he suffers after the shooting of his dog.

Crooks dreams of one day being able to speak to other men, without them judging him for his race; Curley's Wife clings onto her dream of being in the movies, with "with all them nice clothes like they wear", because she is not happy married to Curley, an unloving man

Unfortunately, none of these characters will reach their dreams, proving the harsh reality of the American dream: "An' never a God damn one of them ever gets it [the patch of land]"

Women serve one of two roles in Of Mice and Men: a motherly figure, or a sex symbol.

The only two noteable women in the novella being Curley's Wife and Lennie's Aunt Clara. 

George sees women as an effective "poison", warning Lennie to stay far away from Curley's Wife, as he'll only get into trouble. However, does George warn Lennie to stay away for the sake of her or Lennie?

Curley's Wife's flirtatious nature is ultimately her downfall, and we do initially see her as 'tramp'. However, in Chapter 4 we see her blatant dissatisfaction with life: being married to an unloving, violent man who cares for nothing but his masculinity and pride; with nobody to talk to.

She is never free throughout her time at the ranch, having to move "quietly" so as not to be seen. The only time we see her experience freedom is in death. Where we see her as " pretty and simple... sweet and young"