My personal experience is that depression (for me) is caused by several key triggers:

#1: Extended exposure to stupid behavior

#2: not enough physical activity (another spiral)

#3: not enough social time (stewing by myself)

#4: people telling me what to do (including what to eat) :-)

#5: frustration at not accomplishing my goals

Ok, #1 and #4 are not really depression triggers, more annoyance and irritability triggers. Had to toss that in - please read the sarcasm.

I cure my depression by hanging out with friends, getting out and doing things, exercise, and getting things done. Isolation makes all these things worse. And it too is a spiral for me: I'm "fine" by myself...the more "fine" I get the more effort it seems to take to get out. The comfy chair or get up, get dressed, pack the right stuff, get in the car, drive down the hill, deal with parking, crowds, and so on....the comfy chair is very attractive.

I think there is a physiological link: activity and emotions affect hormones too. When doing those things that make me feel better, I'm sure you'd be able see it in hormone levels. Which supports your spiral too - good hormones == good feelings == good hormones and the inverse is the spiral.

I must emphasize the "for me". There are other causes of depression that are physiological, chemical, and so on. I an lucky that when I'm down I can get up.