It only depresses you in the sense that when you don't have it, you're in a state which is not normal for you. Most of us are used to being high most of the time. If we're not, we're just not feeling normal or not feeling the way that we'd like to be feeling, and that depresses us. Also it's the fact that marijuana sort of supresses negative behaviour and negative thoughts, and when we're not high, we're very vulnerable to those feelings. Also, as somebody else mentioned, it has to do with just having weed period. I know that when I don't have weed and have no way of getting it, it puts me in a bad mood because I'm deprived of something that is very important to me. It's not that I rely on it or anything, but it gives me something to look forward to when I wake up in the morning, and simply put waking up and not having it or knowing that I can't get it, puts me in a bad mood. Just my 2 cents.