Confidence is widely seen as a positive virtue. We like people who are confident, especially here in America, especially here in Silicon Valley. (Some might even call us arrogant)
Yet in my own experience, confidence doesn't always win you points with me.
Confidence is a dual-edged sword--it can cut both ways. The key is credibility.
If you're confident and credible, I'll find your confidence infectious.
If you're confident but lack credibility, I'll find your confidence delusional.
Credibility generally comes from relevant past experiences and achievements. This is why when Steve Jobs did things, he was a genius, and when a first-time entrepreneur who hasn't done anything yet tries the same tactics, he's an asshole.
What this means is that you should only act confident when you have credible reasons to feel that way. Not only will this maximize your ability to persuade others, it also happens to be the right way to act!