Why do we cover our eyes during the Shema?

Greetings to you. The simple reason, as explained in the Shulchan Aruch, Orach Chaim, siman 61, is that it is customary to cover the eyes during the first verse of the Shema to avoid looking at anything else which would distract from proper intention. Because without proper focus, the fulfillment does not count, it is therefore customary to cover the eyes so that nothing interferes with concentration. The sefarim of mussar also write that there is a hint here—that we believe in Hashem even with our eyes closed, and when it is completely dark around us, we still recite aloud, "Shema Yisrael Hashem Elokeinu Hashem Echad." Indeed, customs differ regarding this matter; the main point is that the eyes should be covered. Regarding concentration in prayer, the service of prayer is among the most difficult spiritual tasks. The Gemara relates that one of the Amoraim would confess that he remembered to bow during the modim prayer even while his thoughts wandered elsewhere. Certainly, this requires effort and preparing the heart before prayer, taking a moment beforehand to think, "I am now approaching the King of Kings to request all my needs and to cleave to Him." And even when the thoughts drift, one should strive to return and focus. With Hashem's help and special attention, you will merit greater concentration during prayer. May you succeed, and you are blessed, daughters of Israel, to ask these serious questions. 

With blessings - Binyamin Shmueli