And an angel of the Lord suddenly stood before them, and the glory of the Lord shone around them; and they were terribly frightened. 10But the angel said to them, "Do not be afraid; for behold, I bring you good news of great joy which will be for all the people; 11for today in the city of David there has been born for you a Savior, who is Christ the Lord.…
In my personal opinion the angel was calming the shepherds by telling them not to be afraid of their presence and that they are bringing great news and not anything bad. I believe that the angel was recognizing that they were frightened by the sight of such a heavenly presence and so the angel was assuring them that they have no reason to be afraid since the angel was there to spread good news of the birth of Christ and not to harm them. I don't think that it was a general commandment to not be afraid of anything from now on moving forward, but it's ok to debate interpretation, it's what makes faith stronger in many ways.