I just realized the major cause of my "suffering" from my PhD lies in the fact that it is such a long period commitment, and it is bilateral: me and my superviors and colleagues. I used to have a hell-like PhD in the past (I just started a new one) that my (only) supervisor somehow managed to get into a deep prejudice against me! I know it may be an extreme case, but the (attitude) change of supervisors and colleagues do seem to happen a lot! my PhD is heavily dependent on others' work and honestly, without the strong support of the supervisor, I cannot easily get my degree. If it were a normal job, you can just leave and start over; but nobody wants to waste the time he/she spents on the PhD. I really like research, but probably not do it "professionally"..