It really depends on your situation and on the work you're doing. If you have a project that you honestly care about that you think is really good then you usually want to see it get done. So you might make the decision to overwork yourself even if you objectively shouldn't. And bosses love to set that up, so they'll give you a bunch of annoying crap that you don't think is valuable. But they make you do it and then you end up having to work later in the evening to get done the stuff that you actually care about. Of course, that's something that we all need to be very careful about, but in the short run it's hard to stop.

And another situation that's even worse is that you might lose your job. Sometimes telling the boss no means that you will be unemployed and if you need to make that money to pay the bills then you might be f*****.

I don’t understand why you wouldn’t just continue working your normal hours.

Some of it comes down to real social pressure. People work harder when they've literally got the boss at their desk saying "We need to hit Deadline X or Consequences Y will happen". For people past their breaking point, I tend to see them work less. If you're already on the job hunt (or if you've landed a job or queued for retirement or whatever), enthusiasm for doing your current job plummets. If you think you're about to get fired, same.

But for folks who genuinely believe they've got a future at the firm - at least for another year or three - it often boils down to "Do I want to be stressed forever, or just get over this hump and survive until things die down?" Hitting the deadline and getting the project over the line typically comes with a refractory period of sorts. A slowdown in work hours and a more relaxed pace. Missing the deadline means even more work and even more stress and even more of my boss at my desk (or my boss's boss or my boss's boss's boss) staring at my computer and asking why the thing isn't done yet.