There’s a bunch of physical and psychological aspects that make it feel good (I would say amazing).
The most important thing to make it feel good is to be relaxed and to learn how to control your body to make your hole as receptive as possible. It’s like learning any physical activity. You don’t instinctively know how to swing a golf club, or do a particular yoga pose. But once you figure it out you can just do it without thinking about it.
If you’re tense and tighten up it’s going to hurt. If your top is an asshole (no pun intended) or doesn’t know how to fuck someone less experienced, it’s going to hurt.
Once you figure out how to control your muscles down there, and control your emotional state so you’re relaxed, receptive, and WANT it, it feels mind-blowingly good. And that doesn’t take long, it could even happen the first time. After a few fucks you’ll have it figured out and then just bliss out while you take dick.
As for why it feels so good, my thoughts are the same as other responders. I can’t overstate how good it feels to have my prostate worked over. I like the fullness, I like the stretching. On the mental side I enjoy the feeling of submission, though for me what’s even more powerful is the feeling of opening myself up, and presenting my hole to guys to enjoy, to make them feel good, to give them pleasure, and to get them off.
It’s one of the reasons why, if a guy is getting really deep with me (again no pun intended) I think of myself as aggressively submissive lol. I really really love dick, and I get a huge rush from getting a guy off. Making a man cum feels even more satisfying to me than coming myself. So combine that with the physical pleasure... it’s a wonder why I ever close my legs lol.
Once I started bottoming, which was early in my sexual experience, I quickly realized it feels so good that I consider my hole my primary sex organ. I don’t have the desire to top, and I rarely say no. If a guy wants to fuck me, I enthusiastically give him my hole. If a guy is super horned up and really needs to get off and unload, I love to give him my hole to use.
Plus once I figured out how to control everything I was able to start use that control to increase a guy’s pleasure, becoming looser or tighter, allowing his dick to feel how hot and hungry my hole is for his cock. And there’s absolutely nothing hotter to me than using my hole to rhythmically massage and milk his cock at the moment he blows his load. It really intensifies his orgasm and makes him cum harder (at least in my experience).
I wouldn’t do all that if it didn’t feel good for me too, physically and physiologically. And because it feels GREAT it’s not hard at all to get into my pants and give it up lol.