This is something my husband and I have talked about. We're 35. Depending on the market, we'll hit our FIRE number in 7-13 years. If we worked until 65, we'd be somewhere between really rich and crazy, absurdly rich by the time we retire. Here's how I think about it.
Reasons to keep working:
Work is fun!: We both have jobs that are both pretty enjoyable and pretty flexible. They pay really well. Hard to walk away from that.
We're constrained by our kids' schedules anyway: One of the things we want to do is travel. But extensive travel doesn't work well with a traditional school schedule. And I don't think we're cut out for homeschooling. So, if we're mostly stuck in one locale for 9 mos a year, why not work?
Sense of accomplishment: I've known a couple people who were really good at their jobs. When they retired at 60-something, you could see their pride in what they'd built. They'd done useful projects, mentored younger people, and become valued colleagues and friends. Work is fickle, but when someone finishes on top of a really good career, it means a lot to them.
Be philanthropists: We already give a healthy portion of our income to causes we care about, and we want to do more. Lots more. Even if we only make it to modestly rich territory, we'll be in a position to make a huge impact.
Reasons to FIRE:
There's so much other stuff to do!: We are both intensely curious, project people by nature. We have a lot of interests and hobbies that have gone unexplored. When you're working, those take a back seat.
Travel: While we'd still have the kids schedules to contend with, there would be summers! Long, glorious summers to see the world with our kids before they move out on their own. It's hard to imagine a better gift to give them.
Focus on our health: We're reasonably healthy, but we could definitely stand to eat, sleep, and exercise better.
Be there for our parents and our family: All of our parents are still alive, but they aren't getting any younger (of course, none of us are). And they don't live nearby. Everyone's doing good now, but in the not too distant future, they will need care. I want to be able to go to them at the drop of a hat and stay as long as they need me without worrying about work.
Find out what I'd do if I had nothing do: External motivations have dominated my whole life. I want to find my intrinsic motivation. What do I want just for myself? It's an important question.