Perhaps I am an outlier, but I really think it was the right thing for me. I hit the end of my undergraduate degree and I felt like I had just started learning, so I looked into graduate school and liked what I saw. I've spent the last 4 years working on my PhD and I have learned a lot, I've made real improvements to human knowledge, and I've gotten a job that would have been impossible with a bachelors and 4 years of work experience.
But the biggest reason is simple and terrifying: because it is fun.