Regret. It has powerful strength to trouble our hearts. Some of our most painful regrets are for opportunities lost. As John Greenleaf Whittier said:

Of all sad words of tongue or pen.
The saddest are these: It might have been!

How many people go under a dark cloud by thinking, even momentarily, of the person they almost married, the investment they almost made, the position they nearly won. But for every person who is filled with regret for an opportunity lost, there is another who regrets a deed done, a word spoken, a relationship consummated. These are the stories of decisions made, of tempers lost, of conversations that cannot be re-called. Here are deeds-sometimes sinful ones, but often only erratic or misguided ones - that have changed the course of a life and have left a person with a crushing burden. "I'd give anything," a man or woman says, absolutely anything, if I could take back that one day of my life." Regret. It can eat at your inward being like the most malevolent cancer, destroying by the inch and the hour.  And there is no surgeon's knife, no radium or chemical that can reach it.

Yet, regret can refine and improve character as only a skilled teacher can do. I venture that there are few great saints who have not possessed a high capacity for regret. Effective regret is the growing edge of godliness. But the key word is "effective!"

Saul of Tarsus knew something about regret. His regret was so strong that it surfaced in the midst of a wondrous recital about the resurrection of Christ. As he listed those who had seen the resurrected Christ, he continued, "last of all…he appeared also to me. For I am the least of the apostles, unfit to be called an apostle, because I persecuted the church of God."

Note: This illustration assumes that Jesus in John 14 is attempting to comfort the disciples for the opportunities they will soon squander by denying their relationship to Jesus.