How are you doing?

So, a few weeks ago, I was posed with an interesting question, so I was curious as to other's thoughts towards an answer. When people ask you "how are you doing?", why is it that more often than not people will answer "not bad." Now, I tend to pause and then say "alright" or "I'm getting by", but at this particular time I answered "not bad." The asker then asked, "Why does it seem like we (as humans/Americans/etc.) always compare things to something negative instead of something positive?" My first inclination was to get defensive and say, "well, that's not how I usually respond," but before I could figure a way to say that smoothly without looking like a total moron, I started to think about the question. I think it's a really good question. I realize people just say it without thinking, but what does it reflect that that even entered the stage of "meaningless jargon"? Is it a competitive thing--we want to be better than someone? Or are we so insecure in who we are that we have to say something about what we are not, instead of saying something actually about us? Just the use of a negative seems a priori worse than something without a negative....
So, does anybody have any thoughts? This is kind of a "give me your thoughts" entry, so we'll see if anyone besides Dave actually reads this thing. :)