When you see someone who is happy, it is normal to wonder why they are happy. Likewise, it seems abnormal for someone to have a big smile on their face for no apparent reason.
In thinking about “Joy” and “Happiness,” some would make a distinction. Merriam-Webster’s Collegiate Dictionary (Eleventh Edition) defines happiness and joy in the following ways:
1 a : the emotion evoked by well-being, success, or good fortune or by the prospect of possessing what one desires : delight
b : the expression or exhibition of such emotion :gaiety
2 : a state of happiness or felicity : bliss
3 : a source or cause of delight
1 obs : good fortune : prosperity
2 a : a state of well-being and contentment : joy
b : a pleasurable or satisfying experience
In each case, the other word is used in the definition. I am not sure that it is helpful to try to see a substantial difference. I think it is more useful to realize that happiness or joy always has a context or a cause; it is a state brought about by an underlying (or perceived) reality. (Pity the person who is happy for false reasons.) A person is joyful or happy because they are in a certain situation or they understand something about that situation. The big question is, will this situation or state endure? Is it temporary or lasting?
I think that a typical child living with affluent parents experiences a high degree of joy on Christmas morning; but that joy is short-lived. A young man who has just become engaged to the love of his life experiences a joy that will surely last longer than the joy of Christmas morning, but even that joy will find itself fluctuating as time wears on and circumstances change.
Only someone who is good, eternal, and unchanging could provide a context for a happiness or joy that is superior and lasting. In Psalm 16:11, David says of God,
You make known to me the path of life;
in your presence there is fullness of joy;
at your right hand are pleasures forevermore.
So, if you are seeking your joy (or happiness), why settle for anything less?