“Never let a problem to be solved become more important than a person to be loved. Friends move away, children grow up, loved ones pass on. It’s so easy to take others for granted, until that day when they’re gone from our lives and we are left with feelings of “what if” and “if only.” Said author Harriet Beecher Stowe, “The bitterest tears shed over graves are for words left unsaid and deeds left undone.”
(Thomas S. Monson, “Finding Joy in the Journey,” Ensign, Nov 2008, 84–87)

Here is the best video I found that explains the difference between happy couples and unhappy couples.