Disagree. In fact, the concept of happy, healthy relationships being something you have to work at is not something I've ever understood. (Full disclosure: I'm not actually married, but we've been living together for 15 years so close enough, right?
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My relationship has always been rather carefree and effortless. That's what makes it enjoyable in my opinion. I don't think I could enjoy a relationship where it felt like a constant work in progress, something I always had to tend to. At what point do you get to relax and enjoy it? That's not to say life has always been easy. We've had our share of struggles but the struggle has always come from outside factors -- financial, unemployment, medical issues, family problems, loss of loved ones and grief. Those have always turned out to be some of the strongest and most loving times in our relationship because we're rallying to support the other.
I've questioned what, specifically, takes work in a relationship. The answer is usually communication and making time for one another. Those are things my partner and I enjoy doing. It's naturally a high priority for us, not something we consider work or effort or have to remind ourselves to do. How happy can a relationship be if those fundamental aspects feel like chores?