#14 Make or break
Remember the scene in "27 Dresses" where Katherine Heigl's character finally kisses her crush/boss and she says it just doesn't feel right? That's because kissing is the ultimate "mate assessment tool" that allows us to find a partner who we are genetically compatible with. That's why first kisses can make or break a relationship. Biology can determine whether two people are compatible or not.
#15 Hormones abound
Did you know that open-mouth kissing allows for the transfer of hormones from a man to a woman? Mucus membranes in the mouth are permeable to hormones like testosterone, which a man can introduce into a woman's mouth through kissing. The testosterone then increases arousal in the woman, which leads to higher chances of copulation!