When you decide to spend the rest of your life with someone, you always assume that it's going to be forever. You say "I do" in front of friends and family and profess your love for one another, and then you start the rest of your lives.
However, "the rest of your lives" oftentimes comes with problems, issues, and hiccups along the way. And, those problems can lead to resentment and wanting to totally leave your spouse.