Has no one ever hurt you with his opinion of you when he/she was being true to himself? Did you resent it? Did you feel angry? Or were you philosophical - that's just his opinion? Perhaps you felt enlightened or grateful?
There are two issues here - if someone asks for your opinion you might feel freer to give it; even if it's unfavorable that might be justified. But there are a lot of folks out there giving their opinions when they weren't even asked, just because they're being themselves.
So you don't have to compliment someone's haircut if you don't like it, but neither do you have to lay your negative opinion of it on someone either.