Every time I had to work over 40 hours/week it was because management decided to make a poor decision. Promising customers a new feature without consulting the developers first or just mis-management of the project over-all.
I will say that the main reason I never applied to any startup was the fear that I'd end up working 60, 80, or more hours a week. I value my free time far too highly.
Not to mention the very well-established productivity declines that are associated with prolonged overwork like that.