1. There are too many mirrors. Even if you manage to avoid staring at someone (or avoid being stared at) directly, the sheer abundance of mirrors provides multiple angles from which the inevitable staring can occur.
2. Where there aren't mirrors, there are windows. This is so that those who are not actually in the gym, and therefore cannot take advantage of the many mirrors, can still contribute to the requisite quota of staring. (A few weeks ago, an entire Hispanic family was lined up at the windows to enjoy the spectacular sight of me flailing around on the elliptical. I tried giving the smallest boy a wave, but he continued to stare without even batting an eye.)
3. People wear odd clothes. Most workout clothes seem to be designed for people who have already reached the pinnacle of physical fitness, not for those who are in most need of a workout. If you're going to be stared at (either directly or via mirrors) by anyone, you'd rather not be stared at by a man wearing a cutoff shirt and/or really, really small elastic pants. You'd also probably rather not be stared at by the guy at my gym who occasionally weight-lifts in a hoodie ...with the hood up, of course.
4. There's lots of weird breathing going on. Enough said.
5. Others have passed this way before... and left their marks. Either someone spilled his Gatorade, the ceiling is leaking, or that's an enormous sweat-slick left behind by someone whose neck is bigger than his head. Either way, you're really, really sorry that you didn't see it before you sat down on the mat.