For me, it was a variety of different things. I never really liked working or even being under people. (Blame it on my rebellious high school years and a bad environment growing up) In my field, if you want to be the boss in R&D, you've a better chance of becoming one by having a PhD.
Throughout my undergraduate, everyone, from professors to my fellow classmates, were pretty much convinced that I would make an awesome professor. Keep in mind, I don't look remotely close to the part of the "college professor" but I guess it was my open-minded personality that made them believe so. It also helped that I do like creating new knowledge and never stopped asking questions, no matter how stupid they seemed at the time.
In the end, it just felt right to go to graduate school and get my PhD so I can do what I feel I should do. Of course, I worry that I won't fit in because I come from an atypical background and think differently than most in my field. But I know that would be more of a help than a hindrance after everything is said and done.