When we fall in love, we can't wait to get in bed with our man, right? Why does that drop off?
I went to my psych teacher who helped with my new ideas about people and sex. I told her that I wanted to interview women about how their men introduce sex. I wondered why women seemed to adore sex early on in a relationship, and then find themselves less interested as time went on. People assume that it is something about women and sex, but I suspected it had to do with the relationship, too. I mean, really, sex feels good. So why would someone lose interest?

Source: http://www.ereporter.com.ng/index.php/more-news/life-style-news/item/1164-why-do-we-stop-looking-forward-to-sex