I had terrible posture when I was a little younger. I was given to slouching down in my seat whenever I wasn't being sharply instructed to straighten up, and, as I reached adolescence, hunching up my shoulders and facing the ground whenever I was standing or walking. Like some others here who have had similar problems, I attribute the latter to insecurity about my physical appearance.
My posture has since improved, and I'm now quite happy to look the world in the eye when I'm out in public places. 