Always be filled with joy in the Lord. I will say it again. Be filled with joy. (Philippians 4:4 ERV)
In modern days, we often equate happiness to joy and vice-versa. Happiness is dependent upon the circumstances. Joy is something much deeper, welling up from within. This is why scripture tells us to be "filled with" joy. When people are "favored" in some manner, by good fortune, luck, or some happening that delights them, they are said to be happy. When people are said to be joyful, it usually wells from a deep sense of contentment and fulfillment. There are subtle differences, but one is very dependent upon circumstance while the other is more of a lasting emotion. A lot of people try to give really deep meaning to one and a shallower meaning to the other, with happiness being very fleeting and joy more of a lasting emotion. The truth is that both are emotions - one can be a little longer lasting because it is based in keeping our eyes on Jesus even when the circumstances aren't all that great.
I think that is truthfully the only thing different between the two. Happiness tends to be short-lived because it is focused on the moment, while joy might just be a little "longer lasting" because we are constantly renewed in that joy each and every time we take time to focus upon Jesus. When we see that individual we haven't seen in ages, we are happy - the circumstances give us a sense of delight or thrill. When they are gone, that sense of happiness begins to wane. Joy is a "renewable" thing - it comes in waves and fills us often to overflowing. How? We take time to focus upon Jesus and his grace, love, peace, and other blessings begin to fill us up. In time, what we are filled with begins to "spill out", and others begin to sense this sense of deep joy we have in life. They may not understand it, but they sense it. I have often experienced a deep sense of joy and had nothing going right in my circumstances simply because I didn't take my eyes off Jesus during those harder times. I may not have been entirely happy to be going through those times, but as long as I kept my eyes on Jesus, nothing and no one could steal my sense of joy.
Adversity will come in life. What we do, where we maintain our focus during those seasons of adversity - this is what determines how it is we will navigate within those seasons. William Arthur Ward says it this way: "Adversity causes some men to break; others to break records." I think the thing that keeps us moving along when it seems like we should be beat down by life is this deep-seated joy of the Lord within. Where his presence dwells, there are endless possibilities, even when the circumstances seem to suggest otherwise. Just sayin!