An advice columns in yesterday's paper contained a letter from a woman who had been happily married for 50 years. She said she recently found out that her husband (who confirmed this) had once (apparently quite a while ago but while they were married) had a one-night-stand that resulted in a child. In her letter, the woman said that this has disturbed her so much that she doesn't know if she can ever trust him again.
It seems to me that we hope to find ourselves in our lovers—to merge with each other. In Existential Psychotherapy, Irvin Yalom makes the point that we have four great fears in life: death, freedom, isolation, and meaninglessness.
This is a good illustration of isolation. No matter how close we are to someone else, we are each separate beings. We never know each other completely.