I know depression usually has a grave cause and astronauts have quite an active schedule. But say their family dies. Depression makes you think and perceive time differently. Surely wouldn’t it become a risk to the mission? You can’t steamroll over it…
Psychological screening likely has a strong bias against this situation. One of the key aspects of selection is personality compatibility and how a person responds under great immediate stress. Astronauts are the people that are calm and totally under control while the craft they are flying is disintegrating around them. That is why they were first selected from the ranks of military test pilots.
People process these things differently too. I am very depressed because of my physical disability, but it is entirely circumstantial and nothing can change that circumstance. The only real issue in my situation is if a person is unaware of the effects that depression can have and it causes negative actions. I have meds I acquired just by asking my general practitioner. I’ve had a couple of bouts of existentialism in a negative spiral. Taking meds dulled me until the circumstances changed. In my situation, this is the only real treatment; just dull the person medically. I’m generally more self aware than most people seem in this respect, but I think you will find a similar type of trait in astronauts. The person’s Machiavellian scale will have an effect on how they process emotions against their functional thought and personality too.
I definitely agree that depression is heavily circumstantial. The times when I’ve had it there was always a clear cause and the depression eventually dissipated once I managed to deal with that cause.