What about if your idea will never succeed? You also need to understand when to stop and move on.

With airbnb, there were rejections, but there were also two facts that makes me believe it was a different story:

* They could already make it profitable by just starting to use their own service

* There was an idea of making the world a better place by doing this. This might sound cheesy but I truly believe airbnb, couchsurfing, blablacar and others have really changed the world and how people travel/interact with each other, in a good way. I'm sure I'm not the only one who can see that.