I've been following this thread for a couple of days, but I haven't yet seen this particular argument (individual bits collated from numerous sources including the French BFMTV Live Feed):
1. Andreas Lubitz was a very intense young man. One of his friends from years ago said that "he would have died if he had failed to become a pilot". He clearly set himself very important goals which could not be missed or changed.
2. He was a perfectionist. This and 1. above are well-known causative factors in depression.
3. His severe depression led to a lengthy period off work. Lufthansa of course knew about this depression: this in itself would have been very difficult for him to accept.
3. His colleagues mocked him for having been a flight attendant for a period.
4. He wanted very much to go long-haul but was not accepted.
5. High intensity short-haul, for someone with Lubitz's make up. would soon become tedious, stressful and unsatisfying (as it did for me, and others I know). He could at this stage be thinking: "I sacrificed all of those years and efforts for this?" (as I regret to say I did).
6. Criticism from training captains would have been very difficult for him. Much more than for an easy going FO.
7. Any or all of the above could have led him to see a lapse back into severe depression as a fate worse than death. See 1. again.
There is a possibility than very driven highly perfectionist young men are not the ideal candidates for this career. I have flown with FOs like this, and it's not much fun, and doesn't make for a good flight deck environment . They often have a rigid view of what is correct and what is not, and rarely relax, which is contagious.
Some airlines put sociability at the top of the list of desired qualities when hiring. I'm guessing here, but perhaps LH doesn't?