Suprise, suprise: Scientists have figured out scientifically that people generally marry people who look like them (thus the myth that when people marry they start to look alike has been confirmed!)
While women look for men with qualities that show he will be a good provider and caretaker, men look for women that look "healthy". Thus, complexion is very high on male mate selection (the complexion thing honestly surprised me).
We are generally attracted to people who have different odor/pheremone types (like blood types) than our own. Immunity is coded into pheremones, thus we increase the immunities of future offspring.
Women are responsible for courtship initiation about 70% of the time, but generally with signals so subtle that the male gets credit for taking the initiative.
When someone is "in love", their serotonin levels dip to the level of an obsessive compulsive measures at, thus the ability to remember every detail about a night with a person you are smitten with.