One of the important points that Athol Kay makes on his blog and in his book is his "sex rank". Simply put it is a number from 1 to 10 that indicates how attractive you are to the opposite sex.
For some reason, people tend to pair up with partners whom are equally attractive. Now this does not mean that some people aren't specifically more attractive to others; a 5 might be a 7 to a specific person. But in general we do see a trend (or at least an expectation) of people of equal attractiveness pairing up.
When a difference in this attractiveness starts to arise however, things become interesting.
The woman of the couple of sixes starts to really work on herself and becomes a seven. Suddenly, she is more attractive than her partner. This is where problems start, because attraction generally only happens with equal or better partners (Would you settle for less?)
Now step back and let's take a look at the death of a relationship: Things were great, even for years. But for some reason, one partner is starting to grow annoyed with the other partner. Little tensions arise, sex becomes nonexistent, both people start to become insecure and show their worst sides. Eventually, they break up and rationalise this by stating all the things they found annoying.
But what Athol shows is that this is not the reason for the relationship going awry. It might be the reasons you both see as the source of the breakup, but what simply happened is that one partner was losing attraction, leading to all the frustration and subsequent breakup.
You can't choose attraction.
You can however help it, by making sure that YOU work on your sex rank and increase it. That sounds abstract, but basically it comes down to : Be a better you. If you're a jobless slob, you should know the way already. If you're unhealthy and fat, start working out and lose weight.
Consider that you're not only doing this for yourself, but also as part of your effort to sustain a healthy and rewarding relationship.
So get your butt out and start doing something. (Like buying his book )