You will sleep soon. What is your last thought before you pass out? What words are flowing through your mind? What pictures are there? What have you seen in the past that you'd like to see again? What does it matter? What voices do you want to hear? What would you like to see last before you die?
I think depression is just what happens when you learn that you can make yourself feel good about nothing. Just stimulating a peaceful, calm feeling without effort is the beginning of laziness and apathy. Contentedness becomes a matter of thought and not action. Depression is something to rely on when one finds that the world is unreliable.