Selfishness
Anita Shreve writes beautiful, insightful novels. This one is about a middle aged man and woman, who are both married to others, and have children. They happen to find each other.
They begin to correspond secretly, knowing where it will lead. The book captures the selfishness of adultery, and the madness that overtakes a person when they feel they are "in love" finally, but married to the wrong person. They'll do anything to be together. I've seen this many times.
It's tragic, all around, as you see that once again, their timing is wrong in relation to each other, with great consequence to their families.
If people would put as much effort into improving the marriage they're in, instead of chasing dreams of unfulfilled romances, there would be fewer broken families. I'm not unromantic, but don't wedding vows mean anything?
Also, if this couple had had cell phones, it wouldn't have ended as it did, but then, it was written in 1993