It's really easy to rationalize that your business will work, and takes discipline to try and prove yourself wrong. This is something I've seen in the lean startup movement, but rarely in business.

Most of the traditional businesses I've seen launch had business plans, and their "market research" section was a bunch of untested hypothesis. Have you ever heard a banker and adviser simply state: "I've no idea if people would actually buy this. Do you have tangible proof?"

Instead, they try to sound smart by blabbering about whether they believe your total market size, growth rates, etc...