People are most likely to get into these romantic affairs at the turning points of life: when their parents die or their children grow up; when they suffer health crises or are under pressure to give up an addiction; when they achieve an unexpected level of job success or job failure; or when their first child is born--any situation in which they must face a lot of reality and grow up. The better the marriage, the saner and more sensible the spouse, the more alienated the romantic is likely to feel. Romantic affairs happen in good marriages even more often than in bad ones.
My wife peaked in her then current job, fulfilled her dream to have her own house, car, steady job before she's 30 yrs old. Our marriage was good, or so I thought... and then here I was in the DB forums.