here's a link that explains some of the science:
What If She Were Always in the Mood? | Psychology Today
a few highlights...
as the duration of partnership increases, sexual desire generally declines in women—while desire for tenderness generally declines in men. Usually the more frustrated partner (of either gender) quite logically assumes he would be perfectly happy if only he could have as much sex as he wants.
when male monkeys were paired repeatedly with the same females (who were always in the mood, thanks to daily hormone injections). Monkey heaven, right? Nope. The males copulated less and less frequently, and with declining enthusiasm, over a three-and-a-half-year period. Not only that, when novel females showed up, these slackers hurriedly rolled into action with their original zest.
So what would happen if your mate were always in the mood? Chances are good that you soon wouldn't be...at least with her/him. The sad truth is that if your spouse isn't having orgasmic sex with you as often as you'd like, he or she could be preserving your union by preventing you from satiating yourself sexually too frequently. This is not an ideal situation, however, because without frequent affectionate contact, the emotional bonds between couples weaken, and, unfortunately, many couples drift into engaging in conscious affection only when pursuing orgasm.
dopamine (the "I gotta have it!" substance) was behind the phenomenon of mate fatigue. As a rat copulates repeatedly with the same partner, less and less dopamine is released in the reward circuitry of its brain.
when a novel potential mate shows up, dopamine surges again. It's the same mechanism that causes you to say "yes" to a sugary, fat-laden dessert even when you're full of turkey and mashed potatoes.
dopamine also naturally drops after orgasm, which plays right into this phenomenon. Our genes can be heartless puppeteers.
Scientists call the tendency to tire of a mate with whom one sexually satiates oneself, while mechanically perking up for a new one, the Coolidge Effect. They have observed this phenomenon widely among mammals, including females. Some female rodents, for example, flirt a lot more—arching in inviting displays—with unfamiliar partners than with those with which they've already copulated. In keeping with this phenomenon, when couples divorce because their sex lives have gone out of sync, the formerly uninterested spouse is often startled by a raging libido when a new lover enters the picture.
Our genes want us primed to pursue promising genetic opportunities even if we risk not "living happily ever after." Even if mates manage to stay faithful, this neurochemically induced dissatisfaction can make them see each other somewhat like another serving of "Hamburger Helper." Sure enough, research shows that spouses tend to find each other more irritating the longer they are married