I can't seem to find any scholarly articles on the subject. I've spent several years studying/researching genetics , so I spend a lot of time thinking about possible evolutionary causes for seemingly strange things like this. I've never actually read a study saying on why removing unclean things from our body is so pleasant, but I'm pretty certain it's genetic, just based on the nature of it.

In general, just about every "natural" action that commonly elicits some sort of pleasure or pain response in the brain is probably an evolutionary characteristic. The "reward center" of our brain is tied to the most basic and necessary survival skills. Just off the top of my head, here are some examples:

• Nutritional food tasting good (sugars, fats/oils and meats premium are energy sources in nature)
• Having an immediate disgust/aversion of toxic foods. (mold, feces, rotting animals)
• Being ticklish. In general, the ticklish parts of your body are the most vulnerable/important areas (stomach, neck, armpits, bottom of feet). If you're sleeping out in the wild, you'll immediately be roused and alert if some predator is touching a ticklish/vulnerable zone, which could possibly save your life.
• Exercising, or "runner's high". Obviously, being physically fit can help you stay alive, so the brain rewards exercise with a nice dose of dopamine. Here is an interesting article on the matter
• Pain. Although it sucks, experiencing pain is extremely useful for survival. There's a good reason that getting hit in the balls is so absurdly painful and unpleasant. It serves as a deterrent from damaging the absolute most important factor of evolution: being able to reproduce. If you can't produce offspring, none of your traits will ever be passed on.
• Sex. It's one of the most desirable sensations we have. If you don't have a sex drive, you don't reproduce, and thus your genes aren't passed on. Intercourse, from an outside perspective, is really bizarre and physically awkward. Even if you're strong, fit, healthy, and live a thousand years, but never have the desire to have sex, your wonderful genes will simply end with you.

I'm really fascinated by this sort of thing, because I keep finding more and more little things that most people take for granted, but have an important evolutionary cause. Also, I've had to deal with moderate/severe acne for a long long time now, so I'm extremely familiar with the satisfaction good pimple popping. The little "rush" that comes from purging my body of that white gunk often overrides the fact that I KNOW full well that it's a bad idea.