There are some reasonable posts here. Obviously some people have a lot of practice giving/receiving
I was only top for a long time - I couldn't imagine it would be anything but agony to have a big cock in my ass. The first time I tried receiving, it hurt beyond description (we had no lube, only a little spit and she wasn't gentle - basically raping me with my consent:D). All the points I read here help immeasurably - relaxing, lots of lube, massage with fingers, going slowly, and even withdrawing briefly after initial pentration - all lead to pure pleasure