What is the reason why we are attracted to a certain skin tone? Why have I always been attracted to 'brown' women?

I never knew the reason why I am attracted to women with brown skin and dark hair. My skin is very white and I have also been attracted to pale women, but I have always been more into women with darker skin with 'persian' green eyes.

What is the reason why we are attracted to a certain skin tone? Why have I always been attracted to 'brown' women?
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