As others have said and explained, there is the issue of weather and adequate warmth in a lot of places...but I don't think that is the issue that livi wanted to address.
If all humans lived in a place where the weather was perfect all day everyday, majority of the civilized world (excluding tribes, etc.) would still wear clothes. I think this comes back to the issue that lives of civilized humans are now based on wants, not needs.
For instance, a deer thinks the following:
I need food
I need water
I need to reproduce with the strongest male I can find so my offspring will also be strong, and survive the winter
An american might think the following:
I want some smoked ribs this weekend
I think I might grab a pelligrino
I may want kids someday, but that chick with no ovaries is smokin hot!
You can extrude the 'want' attitude in a number of different ways to explain clothing:
I want to wear those cavaricci's because they are expensive and everybody knows it
I don't want to go nude because I have a huge a$$
I don't want men to gawk at my perfect body
My left booby is bigger than my right, I want to stuff my bra to make them look perfect.
Or, quite simply, I want to wear clothes because everybody else does...
This is, of course, a gross generalization...talk amongst yourselves
