You date, fall in love, and then (whether you get married in Las Vegas in a civil ceremony or have a traditional wedding) promise to spend the rest of your life with your spouse. The reality is somewhat different. We actually often get divorces; 50% of marriages end up that way.
The expectation is that somehow after the divorce, you will erase the past with a single click of a mouse. You get rid of all that was associated with your marriage: photos, things bought together, and if you date again, even a mention of your ex is a sin.
A more mature way to deal with multiple relationships/breakups/divorces is to simply accept the reality that over the course of life we will share our lives with multiple people and in some form or the other, they will always be part of our lives. Just because your partner is dreaming of her highschool sweetheart or you want to show the video of your second wedding to your fourth wife does not mean that you don’t love your current partner.
I think Brad Pitt gets it right. He tells V magazine that he and ex-wife Jennifer Aniston are bound by “a lot of life together,” and that they “still maintain a deep friendship … that isn’t erased in any way” by their divorce.