In John Steinbeck's Of Mice and Men, Curley is the boss's son and is the novel's main antagonist. He is a "thin young man with a brown face, with brown eyes and a head of tightly curled hair." He is also small, seeing as he's a lightweight boxer and "hates big guys."
The last of the above features is why he hates and picks on Lennie. Candy describes Lennie this way to George: "Curley's like a lot of little guys ... He's alla time picking scraps with bigs guys. Kind of like he's mad at 'em because he ain't a big guy." And Lennie is a "huge man, shapeless of face, with large, pale eyes, with wide sloping shoulders." The narrator describes the way he walks like "the way a bear drags his paws."
So when Curley sees Lennie, he immediately dislikes him. Lennie, however, could never comprehend why Curley could hate him so much. But for Curley, he could pick on Lennie for two reasons: 1. He's the boss's son, and 2. fighting big guys is a no-lose situation for him. Candy describes this desire to fight big guys this way:
"S'pose Curley jumps a big guy an' licks him. Ever'body says what a game guy Curley is. And s'pose he does the same thing and gets licked. Then ever'body says the big guy oughtta pick on somebody his own size, and maybe they gang up on the big guy."
Unfortunately for Curley, Lennie is not the typical big guy. Instead, when Lennie catches Curley's attempted punch, he completely crushes his hand.