"You don't know shit" is a constant reminder that you need to try harder--that you haven't figured everything out. You need to live in a state of paranoia that others are just about to release something greater.

See, some people are irritated or shocked by my self-deprecation. The thing is, I would be absolutely insufferable if I let my ego out. I do it every now and then, you know. Most startups are populated by people who have egos that can rival mine, in part because having an ego that large makes you think you can do it better than everyone else, and in part because investors tend to prefer confident people. (I personally think investors are morons in this regard, but what do I know? Managers seem to agree with them. I have seen my more capable but less confident peers get passed over for me or someone like me more times than I can count.)

For me, Yeah, I constantly need to remind myself I don't know everything, or else I end up doing something really stupid. You know I once got into a mailing list argument with Lee Felsenstein about how to run a computer club? Yeah, I looked pretty stupid. See, that's the sort of thing that happens if I don't keep my ego in check.

I know many people who are more capable than I am in every respect who don't have that massive ego. Yeah, for one of those people (who I think would be excellent founders, some of them.) 'you don't know shit' would be a bad thing to remind them. Yes, for them, you'd probably want something positive and encouraging. But for someone like me? "You don't know shit" is quite appropriate.

Once you appreciate that these things arent much more than educated guesses, you can be a lot more focussed actually determining how people react to your product, as opposed to going off and coding what noone wants.