While I do agree that saying yes is good, in the sense that it trains you to take risks and not be afraid, I think saying yes too much can sometimes dilute your focus. I think, as always, it’s a matter of balance. Don’t just say yes for the sake of saying yes. It’s a good practice to train yourself for risk-taking, but for practical purposes it can sometimes distract, in my opinion. Sometimes the smartest thing to do is quit (or say no to) one task to focus on another.