" Recently, evolutionary psychologist have turned their sights on love and divorce. It didn´t take long to notice that when people fall in love, there´s period of up to three years during which the zeal and infatuation ride at a peak. The internal signals in the body and breain are literally a love drug. And then it beginds to decline. From this perspective, we are preprogramed to lose interest in a sexual partner after the time required to raise a child has passed – which is, on average, about 4 years. In psychologist Helen Fisher´s view, the internally generated love drug love drug is simply an efficient mechanism to get men and women to stick together long enough to increase the survival likehood of their young. "

― David Eagleman , Incognito: The Secret Lives of the Brain