It is only possible to succeed at second-rate pursuits -- like becoming a millionaire or a prime minister, winning a war, seducing beautiful women, flying through the stratosphere, or landing on the moon. First-rate pursuits -- involving, as they must, trying to understand what life is about and trying to convey that understanding -- inevitably result in a sense of failure. A Napoleon, a Churchill, or a Roosevelt can feel himself to be successful, but never a Socrates, a Pascal, or a Blake. Understanding is forever unattainable. --Malcom Muggeridge
If so, this resonates with me. I would add that there's another component to this equation that I found personally valuable: challenge. The challenges that I have faced in professional roles (some unpleasant) have often forced me to grow. During long periods spent away from work, I often came to miss that.
For me that does indeed seem to be the most common denominator of 'proper' happiness.