I think it's a personality thing. I'm looking to start a Masters program next year, so I'm not in graduate school, but I can see myself not being totally fulfilled.
But I've always been like that.
I never really expected to even attend University, as nobody in my family (immediate or otherwise) has done that. I remember being so excited at being admitted to University, but of course that feeling passed. Then I remember being so excited to get my first degree, then that passed. As I am about to finish my second degree, and even now, I am not totally happy. I just think of the negative. The fact that I had to do a second degree to increase grades. The money I spent to do that. The fact that I'm not a 3.8gpa student and higher, despite it. The fact that even with two Bachelor degrees, if I don't do further education, I may have issues getting a job. The fact that I didn't do an honours degree, which means I have no research experience. I'm dissapointed at some of my course selection, which meant I had to do extra courses. I'm also not as happy as I'm an older student.
So I'm really excited at this point about the prospect of getting into a reputable Masters Psychology program, but I see issues there as well. It is a terminal degree/no-thesis, so while I know I can do the job I want to do, I already know I'll be dissapointed that it essentially ruins any hope of doing Phd work.
Don't get me wrong, I have many moments of pride for all the hard work I've been doing, and yes even what I've accomplished (especially compared to surroundings), but I think some of us are just never really happy. Some of us really want something, and then when we get it, we want something more or something different.