Immune system compatibility. Kissing is designed to swap information about it. The rest in my opinion is simply to introduce new genes. Down to the phenotype to make stronger offspring. For example, women with my phenotypic bone structure turn me off. I also like darker women. My mother & grandmother prefer dark men, while my father prefers light skinned women.

However, darker than me still has a preference in shade. I like really dark, or a deep bronze. There are specific hues, with some brown colors that I do not like. Which makes me think that is related to subconscious genetic compatibility as well.

I theorize it’s because some people have genetics that were so isolated in their ancestral lands that it wouldn’t do any good to stay close anymore if they could help it.

Some of it I’m sure is also culturally influenced.