A wise professor/mentor/friend of mine had a favorite saying: "If you're in a happy marriage, get out!"
Most people think that's nuts. After all, we get married to be happy, right? Cinderella defied her mean stepsisters and wore those glass slippers (to meet her Prince--so she could live happily ever after! (Or at least upgrade to softer slippers.)
This is what I think my friend meant: the goal of marriage/committed relationships is not to be happy. . Happy is too superficial, too easy, to describe what loving someone else over the long term requires. And, believing you are in a happy marriage might actually be an early warning sign that your relationship has stopped breathing. Complacency can be the enemy of growth.
It's like the saying, "If you meet the Buddha on the road, kill him," meaning if you think you've reached enlightenment in your life, keep looking because you're not there yet. Just as in loving someone else, there is always another layer to unpeel--a story we didn't know, a mystery to discover--about this person we think we know so well.
Terry Real writes that it is better to be "uncomfortably happy" than "comfortably miserable" in your marriage. Many couples give up and settle for the latter and many others keep hoping in vain for "happy" without the "uncomfortable." To be uncomfortably happy means no matter how content we are with our partner, we know there is always a risk to be taken. A fear to be confronted. A change to be made. A possibility of more intimacy and connection, if we are willing to look at the discomfort of our fears and resistance. It's that edge that keeps keeps our marriages and partnerships and friendships uncomfortable -- and alive.