Happiness comes from appreciating what we have, instead of brooding over what we don't have.
Happy people live and cherish the present moment. Sad people lose hold of the present moment by often regretting over the past, fearing the future and looking at what they don't have instead of being thankful for what they already have.   

Jesus hinted at enjoying the present moment as the way to happiness when he invited people to follow him "The foxes have holes and the birds of the air have nests, but the Son of Man has nowhere to lay His head." 
There is the story of a King who won many battles and kingdoms.  He wore the finest clothes and ate the best of food.  But he was deeply unhappy with life and himself and this bothered him. 
His advisors told him that if he wore the shirt of a happy man, he would experience happiness. So he ordered all his courtiers to travel the length and breadth of his Kingdom and asked them to bring him the shirt of the happiest man they meet.

The courtiers went through the busy alleys of the kingdom, the market place, the entertainment houses and almost every nook and corner of the kingdom. 

Then finally they landed in a park where a man was lying down under the moonlight on a stone bench, singing with much joy. 

They rushed back to the King and said "we have found the happiest man in your kingdom.  But he has no roof to live under, and does small jobs just to feed him for the day and above all, he is so poor, that he has no shirt on!"
 This shirtless man tells us, "My happiness comes from a restful surrender to God's Providence each day."