If you are filled with joy then you really have wings, and you can fly. If you are willing to be born, and hang around this world just for this one thing, that is called being happy with this thing. G-d is a merchant selling Shabbos. If it is good to do business with you, he'll make another deal. There is such a thing as emet , truth, and there is such a thing as emuna, believing.
Real joy is a combination of truth and believing.
If emet and emuna and are both working strongly inside of me, if I believe what I know, and I know what I believe, then I am filled with joy. If my job comes from truth and believing, emet and emuna, then it is called holy joy. Otherwise it is called pagan joy. The greatest joy in the world is when a person is really his own judge. If a person can observe himself, know what he did, what he has to do better, that means he is really in touch with himself.