That intense pain you get during... um.. intestinal distress? That's what an enama makes you feel like. Generally, it does go away several minutes after you "evacuate" although sometimes you do have to stay near a toilet for a bit. I personally find the prep much more uncomfortable than the act itself.
The burning only occurs when you use a saline enema (Fleet enemas, for example). For Greek preparation, you're actually supposed to dump the solution, and re-fill with water. If you don't, your ass will feel like a jalapeno makes your mouth feel, and that feeling doesn't go away for a few hours. I learned that the hard way.
I wonder the same thing about doing a lot of anal... particularly, due to the higher risk for hemorrhoids, rectal prolapse, and incontinence.
So why do people do it? Greek can feel pleasurable for the receiving partner. There are a lot of nerve endings in that area, and it's a known erogenous zone. (Note: those same nerve endings also make the recipient potentially more sensitive to pain). Guys also have the prostate, which is supposed to be like the female G-spot. And providers do get higher reviews scores for providing Greek, which is a strong incentive for some.
The thing is, we providers are supposed to make things seem fun and easy. If we b**ch about how much the prep sucked, how much Greek can hurt if not done correctly, etc... that's a total boner killer. For the same reason, if I take a long trip to see a client... I won't tell him how much the traffic sucked, how sick I got of driving, etc. I'll tell him how happy I am to finally see this part of the state. I'll tell him how nice the views on the highway were (even if I only got to spend so much time looking at the view because traffic was bumper-to-bumper for 2 hours). I'll tell him how happy I am to see him, how honored I am that he made the effort to plan our meeting. Etc., etc. And I'll also make an effort to relax, forget the stress of my drive, and put myself in a good-time type of mindset. The important thing is to make your client feel happy and appreciated. If I can't do that, he certainly won't have fun.