You may have a vision for creating a product or service that you believe requires starting a company, and you believe you have a compelling business model that will sustain the company. Failure may happen, but the drive for getting that product or service out there may overcome fears of failure.
The world may be a different place had Steve Wozniak decided to remain an engineer at Hewlett Packard's calculator division instead of creating a company to sell the computer he worked on as a side project, though it did take a lot of convincing from Steve Jobs.