Remember Lady Macbeth? The one who would be queen by urging her husband to murder the king and then afterward was consumed by guilt?

She tried to assuage her guilt by repeated hand washing. "Out, out, damned spot..." and so forth. [Once upon a time, I had a dog named Spot. My father delighted in ordering him out of the house.]

It is a true insight into people that morality and cleanliness are deeply intertwined. If you reveal to a friend that you don't wash your hair every day, you risk being viewed as somehow morally deficient. Filth is immoral; people whose occupations make them dirty are held in low esteem.

OCD people, as is well-known, may take this to an extreme, washing their hands repeatedly, compulsively. There is a sense of relief from the disgust and shame, but it doesn't last long.

For "normal" people (whoever they are), cleanliness is also closely bound with feelings of purity, both moral and physical. There is something wrong with dirt, with the body's sweat and skin oils. Removing them brings relief and reassurance. And tends to self-righteous attitudes.

This is true even when there is no particular odor involved, though that is often given as an excuse.

The other oddity that psychologists have noted is that motivations to generous acts (donations of goods or money as well as helping actions) are reduced immediately after washing.

It's no accident that we talk/read about people "washing their hands of the whole affair". Poor Lady Macbeth. The handwashing didn't alleviate her guilt and she ended up in despair and madness.