It's a heady combination of possession, domination, and sensation. There is also the taboo nature of it that spices it up just right. Another man is letting you literally enter him, be part of him, be used in a way (no matter how lovingly the act is done). It's almost a sacred ritual, and when the lust clears enough for me to concentrate on what is happening in full, I do think of it as such - a fertility rite of the highest order. I feel complete when my cock is buried balls deep into the most intimate part of another man, sharing with him something that only two men can ever share in that exact way; carnal, animal, male.
What heightens my pleasure is the feedback I get from my bottom - moans, looks, caresses, embraces, licks, kisses. Everything he does to tell me that he wants this, lives for this, feels complete himself in this. What makes me cum is the slow buildup up of pressure in his hole as he squeezes me, the exquisite friction of his slick, hot, wet ass tunnel on my thrusting cock, until I can't hold back any longer, and I pour my essence into him, marking him and claiming him as mine.
And whether I have cum or not, I always make sure to stay up his ass when he cums, so that he feels me, and associates my dick with his pleasure.