Physically there's initially an adrenaline rush. Your heart is pounding in your ears. There's an animalistic hunger to breed. When he presses his cock on your hole, there is significant resistance from your sphincter muscles but once he gets passed that first inch, your hole slowly but surely yields to allow him in deeper and deeper. You can feel his cock filling your rectum and pushing into your "second hole". Direct pressure on the prostate feels amazing combined with the stretched to capacity feeling of your sphincter around his girth.
That initial wonderful feeling can fade and feel like too much friction, so regular breaks and relubrication can give your body time to reset and have those good feelings again. I find subsequent entries more pleasurable because your body is already warmed up.
Emotionally, it's the most amazing thing in the world. Being impaled on someone's cock is like complete surrender. It's giving yourself over completely to your top. When he cums inside your ass it feels like destiny fulfilled. Like you've absolutely completed your life's purpose.