Most Helpful Guy
Because you see them being nice to their girlfriend, and how great they are together (hopefully), and can't help but think "I wish I had that." Whereas when he's single, you can't really see what you want as in a relationship so much. He's just a guy that could potentially be in one. When he's IN one, you can see how good it would be, and naturally want in on the action.
Most Helpful Girl
Because people want what they can't have