What is early retirement to me, and why do I prefer it over simply being FI?
It's not having to wake up when it's still dark in the winter, and being able to spend all day outside in the summer, taking advantage of the sun. It's going for long walks up hilltops and not having to hurry back down again. It's spending the night stargazing, looking for meteors, and nobody complaining or even really caring when you get up at midday the next day.
It's having the time to properly clean and declutter my living space, rather than having to fit that in round the edges. It's being able to spend time writing fiction, philosophy, or poetry without worrying about whether it's marketable. It's being able to pursue every intellectual whim I have at the time when I think of it, including switching between areas before I've "finished" exploring the topic by some arbitrary standard. It's spending a week binge-watching some show or reading some book series, with no pressure to get back to "real life". It's having myself be the only person dictating how I spend my days.
It's having no responsibilities or obligations except the ones which I choose to take on, which would be the ones I feel morally compelled to do or feel incredibly inspired about, and which would be ones I could easily drop if I ever changed my mind about them. I want full freedom, not just a little more wiggle room in some corporate lifestyle. That's why I pursue ER.
That said, it is important not to focus too much on the ER aspect at the expense of the present. If you can't enjoy the sunlight on your face now, if you can't find the delight in every new flower in the spring, if you aren't making good use of the smaller amount of free time you get in the present moment, how are you going to manage later? I think in some respects, we have it backwards: ER doesn't bring freedom, but rather, being a free person in your mind already is what motivates ER. And I think it's not necessary to pursue a full-tilt 5-10 year ER, not if your personality is such that being in full-time work for even that long would cause you burnout and stress and health problems. I myself am pursuing FIRE through a mix of part-time work and self-employment. That gives me the chance to do some of the things I would like to do now, rather than put them off until later. You don't have to kill yourself to reach FI, and by taking a route that goes via part-time work or semi-RE then you get the opportunity to find out what you would really do with freedom before it's all you have.
To me, ER isn't about simply setting the auto-pilot investment going, piling up the money, and grinding through, until, DING!, you've hit the arbitrary FI number you set. It's about disconnecting the concepts of "having fun" and "life purpose" from the concept of "spending money". It ties in with simple living, with certain varieties of minimalism, with stoicism, and with sustainability. It's about freeing yourself from what you need to do so you can think about what is right to do and what you'd enjoy doing.
If your living expenses were zero, you could easily retire right now, with a stache of infinity times your withdrawal rate. Just think about that.