Life is a mystery, and this is because no one truly understands life wholly. We live visualizing solutions to our problems and feel if we have this or that, we'll be fulfilled , but realize later on that there's always a new challenge after achieving what we had envisioned to be the solution to our unhappy life. Life just happens to anyone, it either comes with luck and chance or misery and suffering. No one way to happiness, because what works for you may not work for another and vice versa. So it's always a try until we get in touch with that which paves way to happiness and greatness, even in this, we are not truly fulfilled.
What I've discovered over time that gives fulfillment in life is contentment. One who isn't content would never find fulfillment in whatever he gets, because it's never enough. Life treats us differently, but content remains the same. There are people who are happy in little ways and there are also people who in their abundance are sad. Fulfillment doesn't come with what we have or the possessions we've accumulated over the years, it might give some level of peace but itself doesn't make us fulfilled.
The preacher(Solomon) in his Wisdom and abundance regarded all we've labored for as vanity. He concluded that all we have to enjoy is that which we eat. No wonder , people could find joy in Just having enough to eat for the day and those who still pursue wealth wallow in pain and unrest. The concluding part of the preacher's word encourages youths to do what their heart desires without holding back and this is where fulfillment comes.
Fulfillment is doing that which you enjoy doing. The song, "The night" says we'll leave the world behind one day but encourages us to live a life that we could be remembered for. Do everything that makes you happy, it doesn't matter what it is as long as it's not evil and for me, putting a smile on the face of others is that one thing I love to do and that one thing that makes me fulfilled.