I think it's the fear or worry that you'll never know if you married/are dating the right person - if you could be happier with someone else, if life would be "better" with someone else (OCD takes advantage of the word "better" because it can mean just about anything; prettier, kinder, etc.) If you are completely honest with yourself, you'll come to the conclusion that you'll never know if another relationship would be "better" or make you happier. It must be this fear of not ever knowing that can make this obsession so scary. You'll never know this because everyone has different and strengths and weaknesses and no one is perfect or will ever be. Moving to another relationship also means that you would lose all the happiness you have in your current relationship. What you deem the most worthy quality in a mate may not be the same tomorrow or next month when your needs change. REMEMBER, when ROCD has more of your attention, it has you...
A very personal blog about how I've used mindfulness to treat my OCD.