Real talk, there are actually several answers to this question. Everyone who has depression will have had at least one incident where their depression either directly or indirectly caused an issue in their life that they are afraid will happen again. This fear often results in people with depression overachieving as to not feel that horrible feeling again and to hopefully never "fail" again.
Another issue is that people with depression never want to let others down. Knowing the feeling of depression often causes them to be sympathetic towards others as they don't want others to feel the same way. This often causes people with depression to carry the weight of the world on their shoulders because they simply want to ensure no one else has to take that burden.
There is also the issue of people who take up every single task and help everyone in the hopes that a busy body and mind won't have time to be depressed. The idea that with no time to think, there's no way to sink. Of course it then it breaks people down and fall into the other two and back into the third.
Depression is a serious mental disorder that just fucks with the mind.