A key part of this was that, for me, depression is linked to lack of purpose. So there was this question hanging over me, 'what is the purpose of it all'. Well the answer to that question was then essentially 'why are you asking?'.
Putting it less glibly, if you are unhappy about your life, and need some purpose, then introspection about your unhappiness (exactly what are you unhappy about?) can be a way to find for yourself a kind of implied purpose.
And then, another part of it was the realisation that depression can actually make you more powerful, in some ways. To explain this a bit more, if you find yourself in a state where you are not so much able to be motivated by simple desires, then you are also not constrained by them. So, you are able to do do/achieve things that you might not normally be able to do, because you would get bored, for example, or be distracted by the need to do things that make you happy.
And I kind of liked the idea, and aesthetic, of 'just getting your head down and moving stolidly forward'. And, 'if you can't find something to do that will make you happy or interested, maybe you are then free to do something that is right'.