Not the emotion itself, but the idea of everlasting and pure joy. What is it to you?
I just finished watching Vanilla Sky. It's a beautiful story about a narcissist. He relies solely on his good looks, money, and inheritance as a source of power and "happiness," as he perceives it. His happiness is that of which has no real consequences; he takes as he pleases and never experiences the pain of rejection, failure, or loss. When things start to go wrong because of the consequences of him living so carelessly, he escapes into a literal dream world despite his friend's constant warning, "You can do whatever you want with your life, but one day you'll know what love truly is. It's the sour and the sweet. And I know sour, which allows me to appreciate the sweet."
In the end, our protagonist realizes that life is much sweeter and deeper with all its misery and sour moments because it is thanks to them, that the happiest and sweetest moments are able to exist. He decides that he would much rather live and experience all the pains life has to offer, that goes hand-in-hand with happiness, than in a perfect dream, where nothing ever goes wrong... when nothing goes wrong, how would you know when something is right? what do you have to compare it to? One cannot exist without the other.
His last words are, "I want to live a real life... I don't want to dream any longer."
So what is happiness to you?
Happiness to me is not something that you can look for because you won't find it by searching for it; it already lies deep within you. In you is all the happiness in the world. Happiness is accepting the present moment, accepting that you are perfect as you are, at this very second. It is knowing that nothing lasts forever, which intensifies each second and trivializes the future because the future will soon become now, and now will soon become the past. The very essence of life is impermanence, so try to live in the now and appreciate this moment. See it for what it is; passing, ever changing.
If it is sadness that you feel at this moment, know that it, too, shall pass. If it is bliss you feel, appreciate it and be grateful for this very moment. Everything must go. Everything turns into the past.
But what happened to you in the past is not happening to you now.
"We enjoy warmth because we have been cold. We appreciate light because we have been in darkness. By the same token, we can experience joy because we have known sadness." -David Weatherford