Relax men. This is more a woman and kid thing, but there is the odd case where a man has been afflicted. We all know kids hate taking a baths so the phobia of Ablutophobia is rarely diagnosed, if ever.
It's often linked to a traumatic experience, remember Alfred Hitchock's Psycho, the shower scene. That would constitute a traumatic experience. You can understand why someone wouldn't want to take a shower after something like that happening, assuming that person lived.
It can be passed on, mentally. Like monkey see, monkey do. Your mother might have been afraid of cleaning, and you picked it up from her. So you don't shower, often because she doesn't, and doesn't pressure you too, either.
To some people, showering and cleaning is a necessary part of daily life. Society declares it so. It is the first defence against disease and avoiding infections.
Ablutophobia is one of those fears that cannot be dealt with by oneself. A therapist is necessary. If you need help, check Finding a Therapist, first.