Why does Curly attack Lennie?

Curley attacks Lennie because he assumes Lennie is laughing at him when really he is laughing and smiling about the ranch him and George dreamed of. Also because Curley goes after big guys and he thought Lennie wouldn't fight back so that's why he punches him and makes his nose bleed but then George tells Lennie to fight back and Lennie ends up crushing Curley's hand.