I think it seldom happens with couples that at one point in time they start thinking like this but it all depends upon developing maturity in a relationship.
I cannot tell you why I regret my marriage because I do not regret my marriage. When I first got married, I was in my early twenties. We both had very good jobs and we made a decent living. We worked hard, raised three kids and we even had a vacation home on a lake. We are now in our sixties and still I have no regrets.
I don’t feel like I missed out on anything. I’ve had a very full and rewarding life. My husband is a good man. He has always treated me well and I am thankful for him.