We are naturally affrighted at sudden and unusual things, but especially at any Divine appearances, whether God himself takes a shape, or authorizes an angel to do it. So was Daniel, Daniel 10:7,8; and Manoah and his wife, Judges 13:20; and Paul, Acts 9:1-18. For though God doth not make these appearances to affright us, yet such is the imbecility of our natures, that we cannot but be shy at them, and start from them; and it is but reasonable that God should by this means both declare his own glory and majesty, and also humble his poor creatures, and make them more impressive, and receptive of his Divine revelations. It is reasonable God should keep and declare his majesty, though we keep and declare our infirmity.
Related Commentaries of Luke 1:12
Luke 1:12
12 And when Zacharias saw him, he was troubled, and fear fell upon him.