I write a lot about the value of humility, especially in the startup world, but I realized that I needed to add some clarification.
Humility is an important trait, but it needs to be paired with realism.
Humility isn't just self-deprecation. Think of all the times some gorgeous supermodel says things like, "I don't know why people think I'm pretty. I'm just a regular gal." We resent this kind of humility because it seems so obviously divorced from reality.
A supermodel who has spent years on the runway is a trained, cold-blooded professional. She wouldn't be able to stay on top if she wasn't aware of her physical assets, and didn't work hard to maintain or even enhance them. Claiming that she doesn't consider herself beautiful seems disingenuous, and makes us feel like we're being treated like a moron.
Similarly, if someone like the late Steve Jobs were to say something like, "I have no idea why my products are successful. I'm just lucky, I guess!" we would justifiably want to punch him in the face.
There are things I know I'm good at. I can write faster than just about anyone I know (as you can tell by my prolific output). I don't say things like, "Oh, it just comes out of me. I don't think it's anything special." Instead, I say things like, "I work really hard at my writing, and I've been doing so since I was 12."
It ain't bragging if it's true.
The corollary is that it ain't humble if it's obviously false.
Just be realistic about your situation, whether it's your startup, your skills, or your impact on the world. As long as you're realistic, you'll probably seem humble in comparison to the BS artists around you!