In the beginning of learning to bottom, there was always a little pain. I very quickly learned to relax my hole. Nowadays, I only have discomfort if the guy is super thick and/or overly aggressive.
I don't know if you have even done yoga or any other kind of focused stretching. When you get the position just right and the muscles loosen and your body moves a little further in a way it has not before - it's like that except the muscle is your hole. There is this warm, hard presence inside you, and it feels like violation and comfort at the same time.
You know that feeling when you take a really big dump (sorry - not into scat - I just don't know another way to talk about it to someone who doesn't bottom). The stretch is a little uncomfortable but very pleasant. Being fucked is like that except it isn't just a moment of it; it goes on and on the whole time he is inside you. It's pretty overwhelming even after almost 40 years of doing it - intense, stretching pleasure over and over with every thrust. It feels like being brought to the brink of orgasm - not so close that you know the nut is coming but that almost-there point right before - and kept there for as long as it lasts. Even more so if he is hitting my prostate. Honestly if you can do that to me, my mind pretty much goes. I am just lying there consumed by pleasure unable to do much of anything except enjoy the ride.
When he cums inside you, then everything changes. It was all intensity and stretch and friction, and now it feels so warm and slick inside. It is such a feeling of accomplishment to know you were able to get him off with your hole. It feels so good to have his seed inside you.