How ya doin?
Keep in mind, this topic applies to random people you pass at work, out shopping, in a bar. Different response rules apply to closer friends or family. When the average person on the street asks "How ya doin?", it is more like a pleasantry. They don't want to hear about how you had no sleep last night and your bowel movements are not very consistent. They are asking in the equivalent manner of saying "What's up", "How's it goin?", or "Hello".
This is why a blanket response is necessary. Call it insincere, but when I pass someone in a public place, or see a co-worker at work and say "How's it goin?", I don't want to hear their personal problems. Ultimately, I don't really want to know the answer to my question. The people that freely share their personal life's events with people they don't know terribly well makes me uncomfortable. As if I am supposed to offer up a piece of my life in exchange. There is no exchange, I was just being polite.
I feel myself going off in another direction now, so I'll leave it at this. The next time you see me and say "How's it goin?", I most likely will respond with "Good, thanks."

