When Curley, a slightly-built young man, first makes an appearance in the novel, his attention is immediately attracted by the sight of George and Lennie, newly arrived at the ranch.
He glanced coldly at George and then at Lennie. His arms gradually bent at the elbows and his hands closed into fists. He stiffened and went into a slight crouch. His glance was at once calculating and pugnacious. Lennie squirmed under the look and shifted his feet nervously. Curley stepped gingerly close to him. (chapter 2)
As we see from this description, Curley focuses on Lennie, and his hands become 'fists', he stiffens and crouches, he looks closely at Lennie in a 'pugnacious', that is to say, aggressive, way. All these actions suggest a man about to start a physical fight; and in fact we subsequently learn that he is a boxer. His entirely hostile attitude to the newcomers, particularly Lennie, is also suggested by the description of him glancing 'coldly' at them', and Lennie, understandably, reacts 'nervously'.
Curley's specific target is Lennie rather than George, for reasons that Candy later explains. Being small and thin himself, he likes to pick on big men like Lennie to try and prove his superiority over them, and being the boss's son he can get away with it. Lennie is also an easy target because of his mental slowness. It seems that Curley feels a sense of physical inadequacy and tries to compensate for it in this way. In any case his bullying tendencies make him a thoroughly unpleasant character; and his unprovoked confrontation with Lennie ultimately has tragic results.