Why am I still doing cycling trips? I can't deny that, over time, my reasons for cycling have changed. I've done cycling trips for sightseeing, for staying fit, for doing something amazing together with friends, for nostalgia, and for the sense of accomplishment. But I'd say that none of those are currently the true reason that would motivate me to do another cycling trip.
Coziness is a word I would definitely associate with a cycling trip in Japan. You're never far away from society, and you can be pretty much sure that you will not encounter a single person that will try to do something bad to you if you're on a cycling trip in Japan. You don't have to worry (well, worry less) about safety, theft or crime there; the only challenge is the primary challenge: that of cycling and fitness. For every single cycling trip I've done I've been overweight, yet I've managed to cycle up mountains 1000+ meters high while carrying four heavy panniers and a backpack full of crap with me. Your body adjusts. You can go anywhere. And Japan brings with it the coziness that you need to not have to worry about all the 'side quests': no matter where you go, there will be a vending machine with an energy drink or a person who will offer to let you set up a tent in his garden (or temple).
Another part of cycling trips that I cannot do without is the freedom it gives you, but you need to plan it properly. By 'plan it properly' I mean: don't plan it at all, and give yourself plenty of time to get to your destination so you can make detours and change the plan as you go along. It's such an amazing feeling to learn about something exciting during the trip and to take a multi-day detour just to check it out, or even to just find a random road and decide to see where it goes, without having to worry about whether you'll make your flight or not.
This freedom is a thing that cannot easily be shared. As soon as you start traveling with another person you'll end up in a situation where you're trying to maintain a schedule, and/or one person feeling like they're following the other. That is actually a great thing, and some of my most memorable cycling trips have been with others, but as I do more and more cycling trips I'm starting to lean towards being a solo cyclist. I definitely would still go on trips with friends though.
This post hasn't remotely done justice to the feeling I'm trying to describe. I can list the aspects, I can try to tell you why I enjoy them, but in the end, you just won't understand unless you do a cycling trip yourself. So, get going!