As Christians, we sure do a lot to hide the joy that Christ gives us, and we let things get in the way of that joy. A lot of us go moping around like we just lost our best friend, or go around angry at some stupid thing or another. However, we should be joyful because of what Christ has done for us.
Galatians tells us that one of the fruits of the Spirit is joy. That means that if we're living our lives how we should in God's will, we're gonna bear the fruit of joy. There's no way around it--if you're in God's will, you'll have an everlasting joy.
Matthew 5:16 tells us to let our lights shine so that others can see it and desire to have the kind of life in Christ that we do. I believe that a big part of letting our lights shine is being joyful. After all, I can't think of anyone who would rather hang around a bunch of sad and angry people than someone who is joyful. That's one thing others should see in us and one thing that will help them desire to be a Christian and have that joy.
Believe me, if others see that you're always joyful, even in bad times, they will ask you about it. And then you have a perfect chance to witness!
So remember all that Christ has done for you, smile, and let His joy shine through!