In Matthew-32, Jesus indicates the importance of keeping one's marital vows. In Matthew -37 Jesus points out the irony of us making vows in the first place.
Here are three reasons why people are so terrible at keeping promises:
1. We say things we do not mean. Often, we give our word knowing full well that what we are committing to may not happen.
2. We change our minds. I believe that most married couples really planned to love each other forever, and that the repentant sinner has every intention to never sin again. But when the context changes, often so do the intentions.
3. We have virtually no ability to effectively determine what will happen, ever. Even if we mean what we say and stick with it, we cannot stop the rain from falling or the sun from rising. Despite good intentions and best laid plans we live one traffic accident or sick child away from breaking our promise.
We must all recognize our physical and spiritual weaknesses, and turn to the God who cannot lie, never changes and wields all nature to achieve His purposes.