When is a Great Idea a Dud?

So you have this great idea for a business, a 'serve the community' sort of business with a bit of a money maker on the side (possibly). You reckon if you lose money it'll only be your own so hey! why not? And you discuss the idea with a few people: your family members are very doubtful, negative, and afraid of you sticking your neck out (for your own good), and your friends say yes! count me in!

So what do you do? Lots of sums...its still your money....and then you start thinking 'well I guess its not such a great idea', 'guess its an awful waste of money', 'I wouldn't be any good at it anyhow', etc etc

Next thing you know you are dumping all that paperwork and dreams in the fire, just like all the other great ideas before it. You grit you teeth, put your head down and get on with 'real' life, which does NOT include daft notions, throwing hard-earned cash to the four winds, or talking to strange people.

But if 'they' always prevent you taking risks, how can you ever know whether one of those great ideas wasn't a brilliant one? And if you want to measure success through engagement with people and helping them to learn new skills and make the best of things, who is to say that isn't a worthwhile thing to do? Just because you don't make a million Euro in a fortnight who is to measure your lack of money as a failure?

I suppose what I am trying to say here is this- how does one judge between good and bad ideas? As long as its legal, and you're not doing any damage, why not try something out? But HOW do I decide when the people who mind about me think I'm just wasting money and time?

Any pearls of wisdom?