It is such a utilitarian thing–so plain. Something we do in the morning and get on with our day, or something we do in the evening to relax before sleeping.
But there is something about sharing a shower with someone that is so intimate–you are both naked, that is a given and that is often sexual, but it is also vulnerable. You are showing your naked body to someone–in good light–and people do funny things in the shower, so it also makes you a little emotionally vulnerable as well.
To me this combination of the sexual, vulnerable, and wet is highly erotic–it is something that can consistently turn me on whether I am reading about it (erotica), watching it (porn), or doing it (showering together/sexing in the shower/masturbating in the shower).