The latest story making the rounds is that he was being treated for eyesight problems.
http://www.nytimes.com/2015/03/29/wo...-say.html?_r=0
I wonder if this might be a key part of the puzzle.
1) Wants nothing more than to fly for a living.
2) History of depression, which he hides from employer.
3) Eyesight problems, which he can't hide from employer and which may result in loss of pilot's license.
4) Above triggers a depressive episode.