Once you fall for someone, their smell can be a powerful thing. Women will wear their boyfriends' T-shirts, and throughout tales in history men have held on to their lover's handkerchief.
Women apparently are quite drawn to men who have differences rather than similarities in their histocompatibility system. They pick it up by smell, and they can pick it up from kissing.
Women have a better sense of smell than men do, and it's even sharper in the middle of their menstrual cycle, when estrogen levels peak and women are more likely to be deciding whether a man's attractive.
We evolved in a tropical climate where the smells of plants and flowers were all around us. We spent a lot of time in the trees with a lot of sunlight and no clothes.