I am reminded of a scene in Breaking Bad that always recurs in my mind. Hal and an old friend are thinking back to their youthful days of vigor and ambition, wondering how they could have been so happy and eager despite having no money or accomplishments.
"You know what we had?" he says, "Momentum."
That's what leads to fulfillment in life - the direction you're headed. It's all momentum. And it doesn't have to be restricted to dumb 20-year-olds. Identifying and chasing passions are what bring lasting engagement. Happiness is fleeting and nonexistent.