The key was that it's not just one thing, but interlocking self-reinforcing things. (There's another book of his, "The Mathematics of Marriage", targeted at academics, where he actually models a bunch of things as mathematical equations.)
The core idea is what he calls "sentiment override" -- positive sentiment override, and negative sentiment override. Basically, your experience of someone is modified by your attitude towards them. When you have a positive attitude towards someone, negative things get diminished, neutral things become mildly positive, and positive things become very positive. Whereas, when you have a negative attitude towards someone, then positive things get diminished, neutral things become negative, and negative things become massively negative.
And it should be clear that these are self-reinforcing: If your relationship is in a "positive sentiment override" state, where neutral things become mildly positive and negative things are diminished, then it's easy to stay in that state. Whereas, if your relationship is in a "negative sentiment override state", where positive things are diminished and negative things are magnified, it's really tough to dig yourself out of.
So he's got a set of warning signs he calls "the Four Horsemen of the Apocalypse", which are indicative of problem marriages: Criticism[1], Contempt, Defensiveness, and Stonewalling. He said that when couple exhibits all four of those when discussing a contentious topic, there's a 95% chance they'll be divorced in 5 years. (And indeed, by the time I'd read the book, all of those were deeply embedded in our communications; about best we could hope for from any conversation was to avoid having it turn bad.)
[1] He has a particular definition of "criticism" which is similar to but not a 1-1 match with the way it's commonly used.
And he's got a bunch of things to do to strengthen your "positive sentiment", to keep you on the positive side rather than the negative side.
And all of this comes from studying loads of marriages -- both healthy and not -- and seeing what made the difference.
Hopefully that's whet your appetite enough to go take a look at the book yourself. :-)
'Mating in Captivity' by Esther Perel is also quite good in terms of realizing whether you can meet the Needs of your partner and what to do if you cant.
The core idea is what he calls "sentiment override" -- positive sentiment override, and negative sentiment override. Basically, your experience of someone is modified by your attitude towards them. When you have a positive attitude towards someone, negative things get diminished, neutral things become mildly positive, and positive things become very positive. Whereas, when you have a negative attitude towards someone, then positive things get diminished, neutral things become negative, and negative things become massively negative. And it should be clear that these are self-reinforcing: If your relationship is in a "positive sentiment override" state, where neutral things become mildly positive and negative things are diminished, then it's easy to stay in that state. Whereas, if your relationship is in a "negative sentiment override state", where positive things are diminished and negative things are magnified, it's really tough to dig yourself out of.
So he's got a set of warning signs he calls "the Four Horsemen of the Apocalypse", which are indicative of problem marriages: Criticism[1], Contempt, Defensiveness, and Stonewalling. He said that when couple exhibits all four of those when discussing a contentious topic, there's a 95% chance they'll be divorced in 5 years. (And indeed, by the time I'd read the book, all of those were deeply embedded in our communications; about best we could hope for from any conversation was to avoid having it turn bad.)
[1] He has a particular definition of "criticism" which is similar to but not a 1-1 match with the way it's commonly used.
And he's got a bunch of things to do to strengthen your "positive sentiment", to keep you on the positive side rather than the negative side.
And all of this comes from studying loads of marriages -- both healthy and not -- and seeing what made the difference.
Hopefully that's whet your appetite enough to go take a look at the book yourself. :-)