Why are disabled people happy?
One of the most difficult things in life is to be unhappy all the time. Most people have too much to do to waste their valuable time on being unhappy.
This applies also to people who seem to you to be disabled. Generally, they see themselves not as disabled, but as 'differently abled.' They are the way they are, and the priority for them is to get on with life. Whether from birth, illness or trauma, they make a new way of life for themselves, to the extent even, that many would not want their fully-abled body back.
Think of the Paralympian - to them, it is an honour to represent their country, an achievement to win a medal. They are not second-rate athletes who cannot compete with 'real' athletes at the Olympics, they are first-rate athletes given any bodily limitations.
A person with BIID has had a disability all their life, although not a physical one. An amputation, paralysis, blindness or deafness would be no more than a "flesh wound."
See also the entry titled Impairment vs. Disability.