Joy is a subject I've been pondering lately, seeing as I have been experiencing the abundance of joy coming in a rather roller-coaster mode lately. Not great low dips, but great high heights, such that I can't contain it, beyond measure--and I can't fully account for all of it (not that the quantity of joy given is something that can be accounted for), so it makes me wonder at the function of joy. Why did God make joy--what use can it be? It seems of all the emotions the least functional, but also perhaps the most sought. I think it is something that God gives as a reward, a comfort, an expression of His presence in a believer's life.
I try to imagine what life would be like without joy, and it's pretty unthinkable--boring, lifeless, unresponsive. We recognize our need for God in knowing our lack of joy and we long for it, long to know Him; we know He lives in us because He fills us with joy. It doesn't have to do with our circumstances (though I think that's something we have a tendency to forget), because even though we may have little happiness per se, we may still have abundant joy.
If I had as much power in my knowledge stores as I do in my emotions, I would be well-equipped. If I also had as much wisdom as I do emotions, I could do anything. But it seems that my emotions are the most abundant of these three things (though wisdom is available from God for the asking, and I ask Him for it frequently). Perhaps this is the purpose of joy: it is God's stamp that He is working, that He sees, that He loves and cares. It doesn't directly empower me, and yet it enables me to rise above circumstances and ponder and worship God rather than just wallow in selfish thoughts; rising above, then, enables me to ponder others and get a proper perspective that any difficulty I face is not the only one out there, nor is it the worst. So joy is a power of sorts, and helps me to soar on eagle's wings, as God has promised; joy enables me to persevere and to stand firm whatever my circumstances, to seek to do His will, to glorify Him in all my being.