Why is she so offended?

My best friend just got a new boyfriend, and no joke hes an ugly piece of work, and she was just blabbing on and on about him so I got annoyed and said "you're too pretty for him, you can do so much better", and she got really mad and actually slapped me (lol) and hasn't talked to me since. It was a compliment, and true, so why is she so pissed? Geez...