Can we really find joy in our troubles? The world says no, but God says otherwise.
Joy is so much more than just a synonym for happiness... joy is knowing that no matter what, God is in control. Joy is being confident with every fiber of your being that you are loved. Joy is resting in the knowledge that you are going to spend eternity with God in heaven.
Joy doesn't mean you're always happy (though it's hard to deny that truly joyful people rarely seem unhappy)- it just means that you know God's got this whole thing figured out already, and that He can use any situation to display His awesomeness.