The Imrei Chaim of Vizhnitz once participated in a vacht nacht (a seudah conducted in order to provide protection a baby boy the night before the bris milah, during which, divrei Torah and keriyas Shema are recited). After all those who were present recited keriyas Shema as is customary, the rebbe told over the following:
Rav Pinchos of Koretz once approached Rav Mordechai, the chazzan of Zaslov, and asked him, “You perform at many simchas and recite Shema at many vacht nachts. Tell me, why do we recite keriyas Shema on the night before the bris?”
The chazzan said to Rav Pinchos as follows: “Chazal ask (Berachos 13a) ‘Why does the parsha of Shema precede the parsha of V’haya im shmoa?’ And they answer: ‘Because first, we accept upon ourselves the yoke of the Kingship of Heaven - and only afterwards the yoke of the mitzvos.’
Based on this, we can explain the custom just fine. Since this baby will enter into the yoke of the covenant of mitzvos tomorrow, we need to first accept upon ourselves the yoke of G-d’s Kingship; so, we accept Ol Malchus Shomayim first by reciting Shema on the night before the bris.”
