The don't. Braces improve your smile. They allow your teeth to be moved into the best functional and esthetic positions. Not cleaning your braces, or eating harmful foods, can cause damage next to or behind your braces (this is what we call "malpatient"). Keep your teeth immaculately clean, be careful what you eat, follow your orthodontic specialist's instructions, and you will be proud of your new smile.
Unknown. Too many questions as to what is meant by "ruining" your smile - have you completed treatment and your smile is worse? Are you in treatment and the appearance of braces are disturbing to you? Are you in treatment and your smile is looking worse?
So, so rare. Orthodontics is meant to enhance your smile. Of course esthetics is very subjective and you may have unrealistic expectations. Talk with your orthodontist about your fears. It is also true that at the beginning of treatment, for a short time gaps may appear between teeth and may be a cause for worry. Fear not, it is normal for some space to develop as the dental arch widens. The gaps will close.