I get obsessed with what I'm working on and have to force myself to stop and go home. I leave work and I'm still thinking about what I've left.

Because you don't want to be the asshole that did not put enough hour, and did drive the project in a wall. Not that it was going anywhere else...

If a project's success is hinging on its developers' willingness to lose valuable personal time for it, those companies better start coughing up equity.

Or even better, they better start coughing up more $$$. As in substantially more. I think software companies should hire full time but pay by the hour. Overtime will disappear or be required rarely. Employees will be happier.

Of course, bad managers, to make themselves look good will assign 1 weeks worth of work for every hour at first. Till they all lose their jobs.

I think it depends where you are in your career. For me, there are times / projects where I find that I am pushing myself and improving my skills by working on them so I may go beyond whats expected by really delving in, optimizing code, etc to better my skills. Other times, I feel the way you do and have little connection to the work.