Some people enter Ph.D. programs because they're passionate about a subject.
Some truly have a desire to become a professor and teach. And some, like me, go to graduate school because they don't know what they want to do with their life. Six years ago, I left a small undergraduate college with degrees in applied mathematics and English literature, but no idea what I really wanted to do. When a large research university offered me a fellowship to study engineering, I jumped at the chance. I began an intense course of study in management sciences, learning optimization, simulation, and economics.