An innocent question, and fundamentally too wide a question for an objective answer.  For me a startup is a new company run by bright, driven individuals that want to change the world.  The see the current set-up as wrong and that they can fix it, and make a ton of money in the process.  They are fundamentally unreasonable people, that face rejection, a lot of rejection.  In fact, the better the idea they have the more rejection they have - good ideas are not easily accepted, they seem to be strange or trivial or shocking.

"People won't do that", "I find that annoying", "who would want that?", are all signs that you are onto something big.

Never underestimate the weirdness of a big idea.