I'll give it to you that I'm not actually seeing depression on this particular list, though I'm well-aware of its prescribability for depression. Here's a quick rundown of what it DOES help with: http://norml.org/library/recent-research-on-medical-marijuana

Weed makes you depressed because weed causes a dopamine rush and your brain will use it as its primary source of dopamine and stop making it on its own so you become depressed whenever you aren't smoking weed and weed will have a weaker affect every time you smoke it making you more and more depressed.