Because
that's just the way you are.

"Characters
do not change. Opinions alter, but characters are only
developed."

-Bengamin Disraeli

This is the theory
that has spawned the empirical breed of 'personality tests',
rather than the projective, open-ended tests which characterise
psychology.

The trait
perspective looks at personality as a cocktail of
one-or-the-other behaviours. They try to explain a person's
behaviour based on wherther thier outgoing or shy, trusting or
paranoid, organised or unpredictable. Tests are used to see which
of the two possibilities for each trait are preferred, and then
gather that information to make a rather accurate explanation of
the person's overall behaviour. For example, using the
Meyers-Briggs test (here is an example), a person is tested on
for traits- Introvert/Extrovert; Intuitive/Sensing,
Feeling/Thinking, and Percieving/Judging. If a person has the
results Introvert/Sensing/Feeling/Judging, they are said to )(,
and if they are Extrovert/Intuitive/Thinking/Percieving they are
inventive, have very active imaginations, have no sense of time,
and start lots of projects- not nesecarily finishing them, mind
you :). In fact, they are overall wonderful, charming
people...okay, so the author put that in for personal reasons.
Did I mention that they're somewhat self-centered? :)

Of course, the
trait theory probably doesn't take as much into account one very
important factor- setting. It has been proven that there are
obvious changes in a person's personality when put in different
situations. The most noticed changes are those between
introversion and extroversion, and Judging/Percieving. One
example is h2g2- People who are normally somewhat shy and
reserved may become outspoken. Another is the difference between
a person at work and home- a person with a very messy home my
keep their office neat.

The trait theory
describes what is probably the closest to the actual definition
of 'personality' in general- noticable characteristics which
makes a person who they are.