When saying Kriyas Shema many have the custom of covering their eyes when saying Shema Yisrael. What is the reason this is done?
see there for more reasons. chabad.org also has some good points on this.
Nefesh HaChaim Shaar Gimmel Perek Beis(1):
Perek Yud Aleph gloss(3):
Perek Yud Gimmel(4):
Perek Yud Daled(5):
To summarize the above: The reality of existence is that the only true existence is Hashem, and it would be accurate to say that we don't really exist (1). However, living with this awareness, or acting accordingly, would lead to major mistakes (2), and Hashem wants us to live according to our perception of existence. The declaration of Krias Shema is the declaration of that truth, that Hashem is completely one and there is nothing else that truly exists (3). Moshe, who was on the level to perceive that truth which is expressed in Krias Shema(5), spoke to Hashem face to face and his eyes did not dim(4).
My conclusion based on the above: We cover our eyes because we are confronting a reality and truth with which we cannot function in this world. Moshe was able to look with his eyes because he could handle that perception.