The angel said to Zechariah: "Fear not!" He said it to Mary: "Fear not!" And now He says it to the shepherds: "Fear not!" It's a natural thing for a sinner to fear. The more guilt we have, the more things we fear: fear of being found out for some little deceit, fear that some ache we have is God's judgment, fear of dying and meeting the holy God face to face.
But even though it's natural, God sends Jesus with the word: Fear not! Hebrews 2:14 says: Jesus became man "that through death He might destroy him who has the power of death, that is the devil, and deliver all those who through fear of death have been held in lifelong bondage." Doesn't this last phrase imply something tremendously liberating for our daily life? If the worst fear--fear of death-has been taken away through the death of Christ, then surely God does not want us to fear the lesser things in life: job insecurity, not having enough time to finish a sermon, having over for lunch someone who can't speak English, failing a test in school, being rejected by your friends, etc. The message of Christmas is fear not! God is ruling the world for the great good of His children. Believe His promises: "Fear not for I am with you. Be not dismayed for I am your God. I will help you; I will strengthen you; I will uphold you with the right hand of My righteousness... Do not be anxious about your life, what you shall eat or what you shall wear... Cast all your anxieties on God because He cares for you ... The Lord is my light and my Salvation: whom shall I fear. The Lord is the stronghold of my life: of whom shall I be afraid?"
And in the place of fear Jesus puts joy. Joyless faith in Jesus is a contradiction in terms. Paul summed up the goal of his whole ministry like this: "for the advancement and joy of your faith." And he told the Philippians and Thessalonians "Rejoice always and again I will say rejoice." Always? Yes. Not without tears of grief and pain. But still joyful. When a friend on mine lost her mother, she cried for about half an hour before she could stop. But as she knelt there by her bed she was not only grieving. She was hoping. And while it is very hard to describe, there was a kind of joy in God and His sovereign goodness that later on at her funeral she tried to express.
So don't oversimplify: it is not wrong to cry (weep with those who weep), but there is a joy rooted in God's rule of love that is never overcome in God's children.