Desire can bring both sorrow and happiness. Desire the married woman who doesn't know your interest or have interest in you and sorrow is eminent. Desire the promotion to the head of your chosen pushes you to work hard and do your best and there is happiness when you reach the goal.
Even in love, which is a long journey often full of trips, small pain, disappointments that ends in a thrill filled love, once found there will still be tests and struggles, big and small. However love, if it is love, endures and changes and even through the small sorrows there is a bone deep, heart deep happiness.
Above and beyond that, without sorrow in the world, in our lives, we would not know or appreciate the true value of our happiness.