"And the LORD uncovered the eyes of Balaam." From here we learn that human eyes are not granted permission to see as He grants the beasts to see, for concerning the she-donkey it is said "and she saw," whereas concerning Balaam it is said "and the LORD uncovered." And why is it so? Because if a human being were granted permission to see the angels who walk upon the road, he would at once be terrified and go out of his mind. "And he bowed down and prostrated himself to his face." For he recognized that he was an angel and that he had come to kill him.
Why God Had to Uncover Balaam's Eyes to See the Angel
Midrash Aggadah, Numbers 22:31
ויגל ה' את עיני בלעם. מכאן שאין רשות לעיני אדם לראות כמו שנותן לבהמות לראות, שבאתון נאמר ותרא, ובבלעם נאמר ויגל ה' ולמה כן, שאילו היה לאדם רשות כדי לראות המלאכים המהלכים בדרך, מיד היה נבעת יוצא מדעתו: ויקד וישתחו לאפיו. כי הכיר בו שהוא מלאך והיא בא להרגו: