It's a form of thoughtlessness, really. Any reasonable person would sit around and think about the problem and potential solution, and identify all the risks and possibilities for failure, and ultimately come to the conclusion that there's no way to actually do this. Unreasonable people go work for a startup and say, "Yes, I can build this and yes people will buy it", and they don't consider the risks and the failure points nearly as thoroughly. Instead, they create.
Now, failing to consider risks can result in some spectacular failures, and startups do fail. Optimism of this type cannot overcome technical impossibilities or violations of the laws of physics and the infrastructure that exists. If, for example, your solution depends on 1ms latency between Japan and Paris, well, it's not going to work, no matter how amazing your idea.
However, organizational risks and social risks that stop many people can be overcome by simply not considering them. If the problem with your solution is that no one on earth could possibly express anything in 140 characters or less, well, that's a social risk. Ignoring it produced a huge success... why? Because no one stopped to consider the risk; they just did it.
I love startups because they gather people who make a habit of ignoring certain classes of risk, and who avoid paralysis due to those risks. My job is to make a culture where that kind of dreaming and that kind of creation is encouraged. I remove the organizational risks and help with the social risks, and let people accomplish wonderful things that in any other place they simply couldn't do. Don't tell us we can't, and we just might do it.