Even if a person does feel comfortable during anal sex, he or she may still need to learn voluntary control over his or her internal sphincter in order to relax it at will. Doing so requires regularly inserting a finger, perhaps in the shower, each day and feeling the internal sphincter. The muscle changes spontaneously and in response to behavior. In this instance, simply paying attention is more important than trying to relax. Anyone can gradually learn to control the internal sphincter at will.
Anal stimulation provides many kinds of pleasure.
The highest concentration of nerve endings is around the anal opening itself. A finger can focus on them especially effectively. When an object or penis is inserted beyond the anal opening into the rectum, other pleasures are involved. The outer portion of the rectum, like the vagina, has several nerve endings. The inner portion responds mostly to pressure.