Why do smart people become depressed?

A guy I liked in second year college, and two guys I liked in middle school struggled with depression as adults. The first guy was in the gifted program as a child and I believe he was gifted mode 3; mode 1 is for those in the 98th percentile of intelligence; mode 2 and 3 are even more rare, the second guy got accepted into the psychiatry program at the University of Toronto, and the third guy memorized 500 pages of new material in 2.5 days for his exams at the University of Toronto and aced his exams. All three of them went through periods of extreme depression as adults, and at least one considered suicide.

Why do smart people become depressed?
Post Opinion