In Chapter 3, George, Lennie, and Candy are sitting in the bunkhouse, when Slim and Curly enter. Curly (the boss' son) has accused Slim of trying to seduce Curly's wife, and was obviously wrong in his accusation. The men in the bunkhouse begin teasing Curly, mocking him. Curly turns to see Lennie smiling (Lennie is not even paying attention to the other men; he is still thinking about the dream house with the rabbits). Curly thinks Lennie is laughing at him, so he attacks Lennie. Curly punches Lennie in the nose, then knocks the wind out of him with a blow to the stomach. Finally, George tells Lennie to fight back, and when Lennie does, he grabs Curly's fist and crushes it. When Lennie finally lets go, he is very upset because he is a peaceful person, and he doesn't like to harm people; his only concern is the rabbits and the dream home.
I was unsure exactly which part of the book you were referring to, so if this does not answer your question, post another with more specific information.
I think Lennie hates Curley for attacking him without any reason at all. At Chapter 3, the characters were at the bunkhouse, having a card game. Curly had appeared and was looking for his wife, and had said that he would be going to Slim to confront about her whereabouts, thinking that he had seduced his wife and performed sexual deeds with her.
When Slim and Curley returned, knowing that he had accused and framed the wrong person, Slim felt victimized and obviously irritated for being maligned and put under the bad light, people started aiming verbal insult against him like arrows. Curley was furious about the so-called personal attack against him and turn his glare towards Lennie, who was laughing about the idea of a time when the farm, along with the rabbits would be ready and tranquility and peace would be restored. Slim, mistakenly thought that Lennie was laughing at the insults directed at him, he could not tolerate any more such nonsense and attacked him, causing blood to spew profusely from Lennie's nose, again another of his "so-called" accusations that prove false, and also attacked his stomach and had cut off his wind, proving to be another insult to the injury.