I must confess that sometimes I wonder that myself, but then as I reflect on that question I realize that firstly, we married well. What I mean is that we were right for each other. We both had the same core values, same core religious perspectives (we don't believe or think exactly the same, but at the core we do,) we have similar goals, similar passions, similar dreams, similar likes and dislikes. And if you were ever around us you would see that in many ways we are very different as well. But at the core we are two peas in a pod, as they say.
Another big key to our happy marriage is that we aim to please each other, even on a self-less level at times, but never to the point where one is emptied while the other is full. I've seen too many marriages that are all about one taking and one giving. Sooner or later the one doing all the giving will have giving all they can give and they are not going to give any more and they walk.
Communication, honest, open heartfelt and raw communication! In counseling sessions I talk with couples who feel like they have good communication but they don't. Because again, one or both are not really being honest, like sometimes hurtfully honest. I recently counseled a couple, we'll call them Rick and Sally. Rick and Sally have been married 10 years, and it's been a pretty good 10 years, though there were a lot of things about the relationship not going well, but neither opened up to the other to talk about it. So as time went on the marriage sank deeper deeper into unhappiness. Then kids came along, life is busy, doing the deal. But still not really dealing with the issues that lay deep within their souls. And finally Rick and Sally fell out of love and want a divorce; why? They forgot each other!
Plain and simple, you forget each other then soon you have forgotten each other completely and sometimes you can't even remember why you married them in the first place.
When I say open and honest communication I mean telling your spouse what you like and don't like. What you want and don't want, what you are interested in and not interested in, what you fantasize about doing, etc. It's important to do this is a loving and caring way, but it has to be said. To go through marriage unhappy, because you haven't told your spouse you would like him/her to dress up before sex, or shave, or shower, or whatever ... then in a way it's your own fault for not clearly communicating. I had a guy in my office who was only having sex with his wife once every other month, but he wanted sex a couple times a week, he was frustrated and entertaining in his mind an extra-marital affair because of it (that doesn't justify it, but that's just what was happening.) So I asked him if he'd talked to his wife about that, he said he had sorta brought it up, but usually when he was mad. I told him to loving communicate with his wife that he desired her and could like to make love more frequently. He did, and she also said she wanted to make love more often too. Wow, that was easy ... just communicate!
If you have tried to communicate and your spouse doesn't listen, or fights back, using blame, justification, manipulation, or flips it back over on you ... then you need to go see a counselor to work it out.