Continuity mistake: When Jennifer Walters is first introduced, her fingernails are unpolished. Later when she is restrained in Dr. Doom's lair, her fingernails alternate between being painted pink and red. Given that she and Bruce were running for their lives, she wouldn't have had time to paint her nails. Later when she transforms into She-Hulk, her fingernails turn green with the rest of her body, but if her fingernails were painted, they should still appear that color, as her skin and hair turning green wouldn't magically remove nail polish.
Answer: The Hulk is a giant brute who smashes everything in his path when he is angry. In Ang Lee's "Hulk," General Ross, played by Sam Elliot, explains it to his daughter.