Curley, a antagonist in the novel Of Mice and Men, is a very suspicious man. He knows that his wife is an attractive woman and that a ranch full of men is not a "safe" place for a woman to roam.
That being said, Curley knows how all of the ranch hands look up to Slim. Slim is indirectly described as being the one on the farm with which all others compare themselves to. Curley is surely jealous of the respect the ranchers have for Slim, as well as, an almost certain jealously regarding Slim's attractiveness as a superior male.
Curley always assumes that if Slim is not with the other ranchers that he is with his wife. Knowing that Curley maintains great control over his wife, he would surely have a problem with her "stepping out" on him with a hired hand.