I have always believed that there is no comparing living together with marriage. There are enormous differences between being a "husband" or a "wife" and being a "partner," a "friend" or a "significant other"; between a legal commitment and a voluntary association; between standing before family and community to publicly announce one's commitment to another person on the one hand and simply living together on the other.
But attending the weddings of two of my three children this past summer made the differences far clearer and far more significant.
First, no matter what you think when living together, your relationship with your significant other changes the moment you marry. You have now made a commitment to each other as husband and wife in front of almost everyone significant in your life. You now see each other in a different and more serious light.
Second, words matter. They deeply affect us and others. Living with your "boyfriend" is not the same as living with your "husband." And living with your "girlfriend" or any other title you give her is not the same as making a home with your "wife." Likewise when you introduce that person as your wife or husband to people, you are making a far more important statement of that person's role in your life than you are with any other title.
Third, legality matters. Being legally bound to and responsible for another person matters. It is an announcement to him/her and to yourself that you take this relationship with the utmost seriousness. No words of affection or promises of commitment, no matter how sincere, can match the seriousness of legal commitment.
Fourth, to better appreciate just how important marriage is to the vast majority of people in your life, consider this: There is no event, no occasion, no moment in your life when so many of the people who matter to you will convene in one place as they will at your wedding. Not the birth of any of your children, not any milestone birthday you may celebrate, not your child's bar-mitzvah or confirmation. The only other time so many of those you care about and who care about you will gather in one place is at your funeral. But by then, unless you die young, nearly all those you love who are older than you will have already died.
So this is it. Your wedding will be the greatest gathering of loved ones in your life. There is a reason. It is the biggest moment of your life. No such event will ever happen if you do not have a wedding.