There are two stark realities I can think of. One is obvious: people grow old and die - and unless the two of you are in an unfortunate accident, one of you is going to end up without the other, 100 percent certain. When we get married, we think "this is it, forever", but it's not forever. There will be another chapter in life, eventually.

The other less morbid reality is that people change. They age (often poorly), and they may change as people. Sometimes it's better. I'm definitely a more mature, assured person at 67 than I was at 30. But sometimes it's worse, and your partner might not be the person you married.

It does take work to make sure that the two of you remain committed to being the best partners you can be. I remember many years ago, my friends and I were admiring (discreetly!) a very attractive woman who was walking by, and after she was gone, one of my friends said "somewhere, there's a guy who got tired of that." That really made me think. Saying "I do" is a real commitment, and you don't want to be the one who got "tired" of the other person.