Why does Curley attack Lennie?

In Chapter Three, Curley is angry at Slim and busy confronting him in the bunkhouse. While all of this is going on, Lennie, who is oblivious, dreams of his puppy, smiling to himself. Unfortunately, Curley sees him smiling and believes he's being laughed at by the big man. Thus, Curley approaches Lennie menacingly... and starts a fight.