1. I actually hate working alone, and
2. There isn't any positive feedback. When you pass your comprehensive exams, or publish a paper (the "big" achievements), you're really just doing what you're supposed to do. You might get a pat on the back or a celebratory lunch with the lab, but really you're just relieved that you're not a bad student who didn't do those things.
Teaching, however, is the complete opposite.