What a marvelous thing a promise is! When a person makes a promise, she reaches out into an unpredictable future and makes one thing predictable: she will be there even when being there costs her more than she wants to pay.
When a person makes a promise, he stretches himself out into circumstances that no one can control and controls at least one thing: he will be there no matter what the circumstances turn out to be. With one simple word of promise, a person creates an island of certainty in a sea of uncertainty.
Lewis Smedes, "The Power of Promises," A Chorus of Witnesses, Long and Plantinga, eds. (Eerdmans, 1994)