"people who only breath through the  mouth"

I know a person who only breaths through there mouth, when i asked 
why, she said she allways has, as far back as she can remember.

nothing appears wrong with her nose, but she is so use to using the
mouth that she cant breath through the nose................so my
question is how common is this............and why did it happen, what
can be done about it

Ask her to cover her mouth with her hands (or duct tape) and I bet she
will breathe. She has too...:-)

I would agree with the above comment. I can think of at least one
reason some people may breathe primarily from their mouth: they have
sinus problems/allergies which make breathing through the nose more
difficult.

Kriswrite

This can be caused by adenoidal hypertrophy.  The adenoids are like
the tonsils only located behind the nose instead of in the throat.  If
they are enlarged, individuals will appear normally, be able to
breathe through the nose, but require more effort, so tend to breathe
through the mouth.  They may also snore and have more frequent ear
infections (usually as children).