Anyone who's lifted weights will tell you that the least fun body part to work out is legs. You're always left more winded, sore, and physically drained when you seriously train your legs. Plus, it's a pain in the butt to get proper form down for leg exercises compared to your other muscle groups.
So you put it off once or twice. And when you do work them out, you don't go as hard as you should. Then life gets in the way, you don't have time to work out all your muscle groups. So you replace leg day with cardio. You figure if you're running or hitting up the elliptical machine then you're working out your legs enough.
Pretty soon though, you're becoming Captain-Upper-Body with zero leg strength and muscular imbalances out the wazoo, and you start to end up looking like one of the unfortunate souls in the photos below.