I've spent so much of my life chest-deep in depression that I can discern between situational and chemical depression. In situational depression, you can give reasons why you feel depressed, like failure, or whatever. Maybe these reasons aren't the best reasons to feel down on yourself, but the point is that you have them. If you were to talk about it with someone, there'd be things to say about it. Maybe you feel like a failure because of work, or family. There are dots to be connected, and the distances between them are reasonable, if not short.
In chemical depression, the melancholy doesn't always feel like your own. It's like contraband you're holding for someone else, except holding it means having to be in it. It's a cloud over your head, raining down on you, and it's harder to talk about with someone. There isn't a lot to say about it. What do you say to someone when they ask what's wrong? It's harder to respond. It's not any one thing. It feels like everything, but not because everything in my life sucks and causes me depression. It's because I am unable to derive any happiness or joy from everything that used to make me feel that way.
Every single day has been a challenge. I start out the day fighting the sadness. Sometimes I make it through the day, but by evening I am numb. Other days, I give in and let the tears flow. I can hardly do anything, or leave the house. Whether it's situational or chemical depression, to be depressed is to be stuck between life and death. It's not death, but it's not living either.