Good question. I can speak for myself so I'll relate my personal story.
After finishing professional school with a doctorate in hand I went from being extremely busy with lots of interaction from classmates to suddenly being on my own, not busy, and in what seemed an insurmountable debt to pay off. I was also supposed to be a "success". I felt like a huge failure in spite of all the hard work. So I was pretty depressed and this lasted about the first six yrs. after graduation. I never considered suicide but I did consider quitting my profession.
The beginning of the cure to my soul turned out to be a change of venue when I moved to New Mexico.
Very much like the main protagonist in Heidi which I've read three times because I could relate to it so well.
So to answer your question, maybe it's unfulfilled high expectations leading to depression?
Somebody posted about high suicide rates in veterans. I once read a book about Timothy McVeigh which stated he had won all kinds of medals, was a great sharpshooter, and model soldier. After he got out of the military he had no real life skills and had to take a job at Burger King. And you know the rest of the story.