I sat back and gave some thought to the question of why pre-anal enemas might be considered relaxing. I have thought of three possible reasons.
First, they do clean the rectum out. If you're concerned about anal not being clean you're going to have a harder time relaxing for it. After an enema you know you're clean so that is no longer a worry. If you don't have a problem with messes, and you know you don't, then this would not apply.
Second, it is an excuse to have some more anal play/foreplay. Not sure why you'd need an excuse, especially if you're getting ready for anal sex, but that could be an answer.
Third, it could be a ritual thing. Humans like rituals - think about all the ones you have, both acknowledged and unacknowledged. An enema would fit easily into a pre-anal ritual, since it deals with that part of the body and has certain things that you generally do the same way every time.