As someone who is involuntarily on pace to bill 2800-3000 hours this year (and our billing year starts well before the calendar year), just thought I'd add another perspective.
I don't think people really understand the mental effect of billing this much. Getting through a busy month or two or even three isn't hard. We're intense people by nature and this is what we signed up for. What we did not sign up for, however, is the fourth month and beyond. That's when all of the things that were supposed to be temporary start to feel depressingly permanent. When you're busy, you tell people you'll hang out after your deal closes, you'll go to the gym after your deal closes, you'll fix this and that and pay some bills and run that errand after your deal closes. But months go by and suddenly, this isn't just a temporary state, this is your life. The things that normal people do are now, for you, a special treat that you get to do in the middle of the night when you can't sleep. Imagine a life where emptying the dishwasher is a special treat solely because it is a thing you are doing in your personal life instead of something billable. When you're in this frame of mind, taking a few days off or a week off isn't enough because it doesn't feel like a return to a normal life; it feels like a fleeting, nostalgic reminder of what used to be normal, followed immediately by a return to the grind.
I'm in an insanely busy practice group right now that is horribly understaffed, so it's not my choice. The amazing part, though, is how easy it is to manipulate associates into billing this much by using the above principle: it's manageable as long as it all feels temporary. The deal is always "probably closing in 2-3 weeks"... for months. "We're interviewing lateral candidates." "You'll have a break after this deal closes." People complain but they'll do it because there's still a light at the tunnel. It should be obvious that there's not actually a light at the end of the tunnel, but acknowledging that is simply too depressing.
The best analogy is that it's like running a race and seeing the finish line, so you start sprinting. But then the finish line magically jumps back 100 meters. You keep sprinting because you can still see it and it's really not that far, but once you're almost there, it jumps back again. And again. And again. And once you finally cross the finish line, the partners look at you and say, "hey, sorry, we're a little short on runners so we need you to run the marathon now. Oh, you're tired? Yeah, that makes sense, and we're not heartless: take a 5 minute break and grab some water!" ::5 min later:: "But you can't possibly be tired, you just had a break!"