I hate working. I'm smart, talented, etc., so I do pretty well in the workforce but I don't like it. I can't imagine doing it for another 40 years. I'm a saver by nature, so I was putting 25% of my paycheck into my 401k right out of school. FI followed pretty naturally when I realized the conventional advice was to put 10% away to retire by 65.
I don't know, I'm pretty young so all of my plans are subject to change. I'm leaning towards not having kids, but most of this goes out the window if I change my mind. I don't have enough money to quit and live in a van yet, anyway. I'm still paying off my student loans. I also keep working because I've got too many plans! There are a couple of big capital outlays I need to make before retirement:
Long term, I want a house with geothermal heat and solar power. I don't think I'll be able to sell my SO on a composting toilet, but that too. I want to raise chickens in my backyard.
I want to drive a Tesla Model S (but honestly, I probably won't; they're an engineer's wet dream and also an utterly unreasonable car).
I'm going to go to art school. This is the biggest delay on my FI plans. It makes the most sense to do it while I live in a major city near highly ranked, affordable state schools. That means no house out in the middle of nowhere for a few years yet.
With the house and 2nd degree paid for, I think my expenses would be minimal. Healthcare is the biggest concern. I'm hoping we move towards single payer. Location is also something of a concern -- I live in an expensive state, but I like both the state and the benefits higher taxes get me. My hobbies are pretty cheap. Painting, writing, gaming, and hanging out on the internet. I'd also like to travel, but I think that will be doable on a FIRE budget if I plan to be abroad for a year or two at a time, and don't have a house yet -- the plane tickets across the ocean will be the biggest thing. I'm aiming for $20-$30k a year through investments, with the idea that earning an extra $10k through a part time job or freelance work probably wouldn't be too difficult. If I had a million bucks I'd retire right now, for sure.