I have a sneaking suspicion that I've posted this quote before, but I've decided I don't care. LOL It's a really good one!

"Joy is composed largely of that confidence which permits self-surrender in the midst of the worries that harass us."

~ A.M. Avril, O.P., The Meaning of Christmas

When we meditate on the Omnipotence of God, for example, we surrender ourselves in spite of our difficulties, trials and worries. When we meditate on all the amazing things God has done through the entire history of the human race, we (again) surrender ourselves to the wonder of His Might (or the wonder of His Love, depending on your train of thought.) 

In these ways, we actually surrender ourselves to joy, because we are giving over our worries and concerns to His Omnipotent Power.