I'm afraid I didn't catch the name of the author or the book but I heard a TV program playing in the background the other day talking about the key to happiness. They talked about the differences in twins when one was a generally happy person and the other wasn't. One phrase stuck with me, "Attitude of gratitude".
They were saying that feeling grateful for what you have rather than always wishing you had something else is the key to happiness. I can understand how that would certainly apply to a happy marriage. I can see how that would lead to contentment. But I'm not sure it means true happiness.
I tend to be a happy, content, grateful person. That tendency also causes me to have very little drive to go outside my comfort zone and strive to succeed in new things. Luckily, I have a very driven husband who urges me to do such things all the time. I don't think I would be truly happy otherwise.
I think the key to true happiness is a balance between being content and being driven. I'm not sure how you achieve such a thing on your own though. Most driven people are that way due to some form of insecurity or need to impress. Most content people tend to stagnate. But that's just my unscientific observation.