They are depressed. They feel awful inside and can't find any happiness in their lives. They meet the wrong person at the right time and suddenly there is a bit of excitement in their day. And it begins to grow...out of control and into an affair. But they don't care about their marriage any longer because they've finally found something that helps them escape from their unhappiness and makes them feel good again. For a while...
Most of us really marry only once. First love endures, even unto our dying day. And we never really divorce.