Stage One: The person who ends up cheating does something to irritate the other person; what's more, this act is blown all out of proportion. We see this when Steve wants to act like a regular guy instead of conforming to the unrealistic standards Annie has, we see it when Eric reminds Liz that she moved in with him and shouldn't make too many demands of him, we see it when Therese wants not to be made a fool of and we see it when Liz piously jerks Warren and Paul around. These things would blow over if the second party were to discuss things openly and honestly but that's too much like work so we have to deal with the Pattersonian way.
Stage Two: This, of course, entails the second person self-righteously inflicting an act of appalling and mindless cruelty on the first. This is why we have to contend with Steve being told to his face that he doesn't measure up, why we see Eric being hectored by a shrill pain in the ass called Liz, why Therese had to stand there frowning while Anthony dismissed and trivialized her feelings of betrayal and also why Liz dangled Warren in front of Paul. Since most people are not Pattersons and do not want to maximize their agony, they seek escape; this leads us to the next part of the cycle.
Stage Three: The second person has an affair so they can be reminded that not all people are as hateful, imbecilic, vindictive, immature and irrational as the Pattersons and their fellow travelers. This state of affairs persists until they are eventually found out which leads us to the final phase.
Stage Four: The first party has a huge, blame-evading tantrum wherein they avoid seeing how their wanton, idiot malice helped drive a wedge between her and her partner followed by breaking things off in an undignified and revolting manner; unfortunately for Steve, he wound up with someone who didn't believe in divorce. This led to an extra phase in his life.
Stage Five: The man is now pretty much a prisoner of the blustering, sanctimonious drip who bleats about practicality when he sees that romance is called for. The only thing that keeps him from going insane and smashing things is the reassuring inevitably of sweet, sweet Death.
Now I know that both parties share fifty percent of the blame for what went wrong; I also know that Lynn cannot see it that way. What Lynn does see is that that act of ridiculously overblown revenge I mentioned is justified by the minor annoyances; in short, Annie had to act like a pissy old bitch because Steve gives his kids cookies because that's how things are supposed to work. That's because she's insane and stupid.