Anything is possible, but in your opinion how likely is a relationship started, when either person is in/coming out of depression, to last?
Does it vary due an individuals nature, how they deal/heal/escape depression? As in with/through another or mostly independently (thinking in terms of relationship oriented/individualistic signs).
I think the odds are pretty good, if the people cheer each other up and encourage each other. I have a lifelong friendship, thirty-five year. When we met, we were both very low.
Depends. Is one person, or both, gotten to the point of full blown psychosis (hallucination/delusions)??
It's horribly complex and specific to any individual and/or couple.
l would think that a healthy balanced relationship should help with depression, all the dopamine your body releases and such.. l think there is no better drug to help someone get out of a depression...
anon, wait, love's not a form of psychosis? 
🙂
True clinical depression is a condition that can be treated with medication and psychological help. Doubtful a person actually in that state of depression could start, get or give anything to a relationship. Clinical depression is not simply being "low or down in the dumps" or in a funk kind of sad, it's not something that can be "cheered up".
Some people are naturally morose, or moody which is a personality type and not the same thing as depression. If that's it, you would have to be accepting of that type personality in order to be in a relationship with them, but it takes two to make a relationship and you can't answer for the other person.