Who am I to answer this question? It's not like I'm even remotely close to having a Ph.D., but still, I've done a lot of thinking on this topic over the past two years (which is hopefully a sensible thing to have done). What exactly does it take to get through a Ph.D. program? I'm not talking about things like intelligence or innate creativity or anything like that. I'm talking more about one's general (and current) attitude towards everything. I am able to condense it into two main "prerequisites" that seem essential:
1) You have to find your niche in the field. You have to truly enjoy (or somehow convince yourself that you truly enjoy) what you have chosen to study. This is somewhat obvious, but worth pointing out nonetheless.
2) You have to not mind being tied down for several (i.e. 5 to 7) years. This one is the kicker. In my experience, there are two different types of students who seem to have this trait. Type 1's are those who don't care about anything besides their work. These types are very common, in my experience. Type 2's are those who have entirely fulfilling personal lives. This is a broad concept, but it is common among those who have stable families or loved-ones nearby. Also included in this characterization are those who are more free-spirited--people who excel at living life one day or one week at a time.
These two attributes are essential to the Ph.D. student. Contrary to what some want others to believe, intelligence and creativity are not the most important factors. Most crucially, the student must simply be a highly motivated individual. To me, these two conditions are necessary components of possessing such motivation.
In self-reflection, the prospective Ph.D. student must ask himself or herself, "Am I at a point in my life where I know (with certainty) that I have both of these traits? Is this what I
truly want to be doing, right now?"