For me, it seems to be multi-faceted. There's definitely some kind of mood swing component that seems entirely biological; regardless of whether things are going well in life or not, there are periods of between a week and sometimes as long as a month where I simply have difficulty getting up in the morning, focusing, socializing, and maintaining healthy habits like regular exercise. Usually my sugar consumption skyrockets when this happens and I keep bad hours and the cycle feeds into itself.

Then there seems to be a slightly greater sensitivity to problems. Outwardly, I can power through a lot of problems -- financial trouble, management, work, personal life stuff, whatever -- but inwardly, it kills me a little bit.

Annoyingly, other people's problems bother me a lot too, extending all the way out to sociological issues. I think this is part of what created David Foster Wallace's famous speech, "This is Water" (https://www.youtube.com/watch?v=8CrOL-ydFMI) -- he eventually committed suicide. But this can also be a strength, it's what drives me to try to make a difference.

Anyway, the real trouble begins when I'm in a biological downswing and things in life start getting really hard. In a single year, I lost most of my friends over a disagreement, my parents both had serious issues (and I was their counselor for much of it), my business was struggling, and the effort I had put into training for search and rescue all Summer got wasted when the vehicle I was driving died on the way up to the final exam. It was just a lot of stuff all at once, so when the mood problems hit, I barely functioned at all for about a month.

The only common theme that I think I've found so far among people who struggle with depression is a feeling that nobody cares about them, or that they're undervalued in some really big way. Struggling with depression is challenging; struggling with it alone is extremely difficult.

On the other hand, because of the social stigma it currently carries, few people want to make a big deal of depression when they're struggling with it.

So, if you think you know someone who's having trouble with it, it can sometimes help them a lot to just initiate contact and suggest spending just a little bit of time doing something they enjoy: a card game, going out for a bit, watching TV, whatever. It doesn't have to get heavy. It'll help them to know that somebody cares.