Lots of people start PhD programs, but far fewer finish (in some programs the drop-out rate is 50% or higher). Some people are motivated to get a PhD by: wanting a job in academia, wanting a job in industry, personal pride, or simply a lack of direction and an aptitude for the field of their PhD program.
Have you noticed that people with certain motivations are more likely to finish their PhD than others? If so, which reasons correlate positively with success? Conversely, do you have warnings for someone considering starting a PhD?
I won't tell you what are and aren't good reasons for getting a PhD. However, to get a PhD, you must stay remarkably determined for a minimum of 5 to 6 years. If you cannot, you will quit. Earning a PhD is hard. When I earned mine, it was easily the hardest thing I had ever done.
During your PhD, you work long hours, for a low salary, with little respect, and bleak job prospects. Most PhD students are at least fairly smart and/or moderately hard workers. Nearly all of them could make more money with less effort elsewhere.
So, why get a PhD? You need to find your own compelling reason. I couldn't imagine not studying math. Undoubtedly, I would be studying it now, even if no one would pay me. I'd never cared too much about money (easier to say when you're in your 20s and single). I was far from balanced, so long hours didn't bother me. I really loved teaching (and still do), and after working with high school kids, I decided I'd much prefer college. So I decided I'd be a math professor. That was my vision, what I clung to in the midst of the storm. And it worked, eventually. Your story will be different. But it must be just as compelling to you, or you likely won't make it.