In the novel 'Of Mice and Men' by John Steinbeck, Curly's wife illustrates one of the themes - power - when she has the cheek, front and sheer nerve to invade a person's privacy and personal space. She should not be anywhere near the bunkhouse or rooms and has no proper business there. Yet she flaunts her power over poor black Crooks by reminding him of his hopeless station in life, and that there is nothing he can do about her actions. She exploits his powerlessness to do anything about her invasion by threatening him with 'framing him' for the very sorts of misdemeanours that Lennie Small is actually guilty of. She has the power of color and staus - even though she is a woman. Crooks remembers why they are all where they are.
To me, the reason that Crooks changes his mind is because Curley's wife has robbed him of all hope for the future.
Before Curley's wife comes into the bunkhouse, Crooks has started to buy in to the idea of going along and chasing the dream that George and Lennie have been chasing. He is finally feeling some hope.
But then Curley's wife comes in and smashes that. She forcibly reminds him about how powerless he is because of the fact that he is black. She reminds him that she could have him lynched just by saying he touched her.
So now Crooks is demoralized. He feels like he is nothing and no longer has the ability to dream of a better life.