Why? My question is why not? I have so many interests outside of what I do for a living. My job is just a means for me to make some money so i can further explore and pursue those interests.
Some people have asked me why I want to retire and what I would do. Here's the why part: one of the gentlemen that I work with is 54 years old. His net worth is roughly 2.5 million, no kids, no debt. There's a handful of other people in the office just like him. One day in the office we were talking about my trip to Vietnam. My wife and I enjoy traveling to off the beaten path locations that are typically very inexpensive. He's always asking me about my travels and where our next trip will be. So I asked him, why don't you just retire now and travel for a while and he replied "because when we're not traveling I'm afraid I'd get bored". And I said "really? I could think of at least 100 things I'd rather be doing right now than sitting in this office" And he said "well why don't you go do those things?" That got me thinking really hard. I compare him to Brooks in the movie Shawshank Redemption. He's become institutionalized. I'm leaving before that happens to me.
I don't rely on my job as a source of self importance or identity. I don't need to keep working to "use my brain" or work for the "comradery". I get plenty of that in the things I do outside of my job. So I hope that answers your question of why I want to retire in my 40's.
As for what I would do: the list is too extensive but a few examples would be, grow food in the garden, mountain bike, paddle board, hike in the White Mountains, practice my guitar, slack line, learn French & Spanish, volunteer for habitat for humanity and the Appalachian Mountain Club, refine my photography skills, learn to draw, cook amazing meals every day, bake sourdough bread, maintain my car and house, repoint my chimney....
I haven't even mentioned the things my wife likes to do. I hope this answers the question of why and what.
This is an early retirement forum, surely most people can understand why I want to retire, no?