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.
If you're a man and you have big tits, don't wear a tight T-shirt, okay? It confuses the children!