There are lots of answers here on OnStartups that imply or even outright state that one has to be obsessed to succeed with a startup. When I see "obsession", I hear "imbalance". Is it possible to be balanced in one's approach to creating a startup? Is it only possible to succeed if it is the absolute tippy top most important thing in one's life?
I'd say drive. I am working nights and weekends on a self-imposed deadline. I have four kids. You have to be driven to do this. My wife does call me crazy.
Insanity is a reflection of the amount of work that is usually necessary to create a real company.
The amount and scope is so unexpectedly huge that it takes an insane amount of work and drive.
There are always exceptions to every rule, but even if you succeed, and once you are doing well, so many people will try to copy you and out-work you, that you will have to work even harder then. It never ends. It is constant stress. If you are not crazy before, you will be crazy by the time it is all over - so that is something to look forward to :)