When we look back and perceive that we were happier "way back when," is it true? Those were not necessarily happier times, just different things about which to be happy.
But the point is well taken that the pursuit itself (doing, discovering, designing, creating, etc.) is what defines this state of well-being. As Longfellow said in his poem A Psalm of Life, "Let us now be up and doing ... still achieving, still pursuing."
It can be found in the lives of tradesmen, businessmen, a housewife taking care of her family, a farmer tending his crops, a writer developing a story or putting together a phrase or poetry to express himself, or people in general who have accomplished something. But they surely find joy in the process itself, even though they may also be rewarded tangibly.
Surely, there is ego involved in satisfaction and accomplishment, but it is the achieving that inspires this feeling. Success comes from venturing.
Often, this pleasure comes from a spontaneous or impulsive act, such as when a person is shopping in a store and suddenly sees something and impulsively purchases it, etc.
A writer of note once penned that "things tyrannize a man" and that "a man is rich in the things that he can do without." But one of the things we cannot do without is happiness. We say, "Ah, sweet mystery of life" when we revel in those moments in life that we catalog as joyful periods.
Happiness, the noun we try to define so, is basically a product that is an activity, rather than a plateau. We climb a mountain, and from that vantage point in life we realize that any step we take is downhill, in a move away from that peak. The peak that a pursuit of wanting to be happy took us to.
So at an advanced age, we will look about from the mountain top for a new pursuit. Using the reasoning, the experience, the learning, which was a happy time.
The teaching of the experience, the learning and so forth, the smelling of the roses along the way, the raising of a litter of kittens, or rabbits, the writing of a poem at eventide, the exploring at the seashore, the cuddling of your newborn child in your arms.
The end result is happiness, but so is the pursuit. There are many paths to achievement, each of which is constructed through such cheerful moments. So now, pursue your endeavors with the resulting smile on your face, radiating all the while, the not-so-illusionary thing called happiness.