The best reason for a woman to shave her head is to prove that she is beautiful. Everything about your head -- including your face -- is so exposed -- exposed to our judgment. Natalie Portman proved her excellence. So did Demi Moore. Britney too.
Don't think that's why they do it? Think again.
IN THE COMMENTS: Some folks say Demi and Natalie shaved their heads because a movie role demanded. They have it backwards. Both actresses -- I'm willing to bet -- chose the role because it would require them to shave their head and they wanted the opportunity to demonstrate to everyone how amazingly beautiful they are.