the #1 cause of pain is your Lonely housewives want seduction horny black mothers instinctive reaction to something coming in from outside. Your sphincter contracts, and forcing it is like getting a knot in your leg muscle. However, relaxing is more easily said than done. The best method is LOTS of foreplay. Your partner can SLOWLY work on your asshole until it gets used to the idea of something going in. Trouble is, that might exceed your partner's. You can try practicing.. get something small to use as a dildo, get comfortable with that, and move up from there. You can try alcohol. Lotsa guys have a drink as part of the social ritual. It help because it relax you. You can try poppers. As a temporary fix, this is bound to work because amyl nitrate both relaxes the muscle AND numbs the potential pain. But poppers also tend to relax another "muscle", and a lot of guys can't get hard for a while after getting a whif.