Most human beings seem innately averse to change. Once we've established some measure of comfort or stability, we want to nail it in place so that there's no possibility of loss. Unfortunately, this is another promise that is more likely to scuttle a relationship than shore it up.
The reason is that everything—and everyone—is constantly changing. We age, grow, learn, get sick, get well, gain weight, lose weight, find new interests and drop old ones. Many people fear that if their love is free to change, it will vanish. The opposite is true. A love that is allowed to adapt to new circumstances is virtually indestructible.