Arresting the fight or flight response
At the seat of everyone’s problem (men and women) is the human “Fight or Flight” response. The human Fight or Flight mechanism is wired directly to your ass (literally). Along with many other parts parts of your body. If you give someone a big enough fright they will literally shit themselves (as well as pissing themselves) uncontrollably. Hence the phrase. This is a very primal reaction designed to lighten the load and eliminate any distraction from your next reaction. Either you’ll be running for you life or fighting to the death. Anything short of “oh shit, I am going to die” will make your ass shut tighter than Fort Knox.
Any trepidation, fear, concerns, anxiety, nervousness, stressed, etc., that is lurking in the back of your mind will tence the mucles in your anus. Even stress not related to the thought of having anal sex. Anytime you stretch or force a clenched muscle you will experience pain. Worst case, you will actually tare the muscle, resulting in aches and pain for days. This stress/anxiety issue is also why you will find anal was easy on pasts occasions and at other times it is impossible.
Any muscle will relax if massaged correctly, usually in conjunction with soothing our mind. Not always the case with anal massage. If someone or something touches your ass when you are not ready for it, your sphincters will clench. This is the Fight or Flight response. Your ass to pucker up and shut tighter than a vice before you even have a conscious thought on the matter. When you are use to having your ass touched, the right touch will relax and open your anus. The first step in learning to enjoy anal sex is for you (and your ass) to become use to being touched.
Being clean is a big advantage
Learning to enjoy anal sex or anal play of any kind usually starts with addressing the issues of cleanliness and hygiene. This does top most peoples list of issues with anal. But cleanliness and hygiene leads to quite a bit of discussion on its own and distracts us from dealing with the neurological issue and it is the neurological issue(s) that I do not see people addressing at all. So I will deal with cleanliness and hygiene separately.
So if we take the position that cleanliness and hygiene has been addressed because it is actually pretty easy and straight forward, and there can be no objection to anal play on these grounds then simply becoming accustom to having your ass touched turns out to be a bigger issue for most.