How many great ideas have you had that never came to fruition? (Whether business, personal project or otherwise)
Of those, how many of them didn't happen because they were met by such great resistance from someone outside of yourself?
If you're like me, the vast majority of your big ideas don't happen because you shoot them down before anyone else can. You don't put them out into the world or test them or find a way to see how viable they are. You dig up the seed before it can germinate. You tell yourself no.
The only way to get past this is to "let them tell you". Don't assume you know what the market is going to say. Ask and be willing to hear the answer. If the answer is no, if no one likes your idea, if you can't generate any interest, then reconsider. But, not until then.
Of course, this applies to so much more than just launching something. It applies to all manner of questions you need to ask, but are afraid to... the raise at work, the date with that girl, the terms of the lease you're signing. Ask. Let them tell you.
I'm not suggesting that "it never hurts to ask". Ever asked your boss for six months of paid vacation? It's not likely you would recover well from that kind of request. But, it does hurt to not ask when you need to.