How is it any different from plastic surgery?

Can you answer AlwaysOnTheBSide’s question about Plastic Surgery?:

My friend has braces and is totally against plastic surgery, but I don’t understand how it’s any different. You’re still using something mechanical etc., to alter your physical appearance.

Opinions?
Okay, to clarify my meaning: if someone has a small gap in their teeth that won’t cause them any pain, and they fix that to make their smile better or whatever, how is it any different from, say, a nose job?

Breast Enhancement Without Surgery