confidence is for marketing guys you should make important decisions on the rational level, not the emotional level. You should weigh the risks and rewards in a rational manner, rather than running with what you feel. Feelings can be quite mercurial, and if you quit after you have a depressing weekend, or (sometimes worse) get in over your head and charge up lots of personal debit because you 'feel good' you can get…
Confidence is necessary for anyone who wishes to achieve anything extraordinary. It'll come in handy when you're working on a dinky little idea that no one else understands or respects. It'll come in handy when you're quitting your day job, driving a crummier car than your neighbors, and working late into the evening for $0/hr. It'll come in handy when you're asking investors for money. Need I go on?
If you make important decisions solely on a rational level, how in the name of all that is holy could you ever start a company? What if your rational decisions never lead you to a 'positive expected return'?