why is slim angry with curley when they return to the bunk house?`

Slim is tired of Curley questioning about the whereabouts of his wife and insinuating that he is messing around with her. He says

'Well, you been askin' me too often. I'm gettin' God damn sick of it. If you can't look after your own God damn wife, what you expect me to do about it?'

Slim is a calm, peaceful man. He simply wants to get on with his work and interact politely with the other ranchers. He does not like that tension that Curley creates.