I don’t think anyone really knows. It couldn’t be pheromones because we become infatuated with certain celebrities that we never meet in person. It doesn’t seem to be the biggest and strongest, as in the animal kingdom, because as @slopolk said, and most women agree, muscle-men model types aren’t often the object of our infatuation. Maybe people do (subconsciously) prefer someone who resembles their mother/father.
Personally, I have found that my “type” is a pleasant-looking, not-very-tall, dark complected man with dark hair and eyes. That would be my dad. But unlike my dad, I am attracted to guys who have an inner vulnerability, covered up with an outer bravado. Also, my guy has to be intelligent. I loose respect for a guy who is not too bright.