One of the reasons why the bris milah takes place on the eighth day is because there is an angel teaching the baby Torah in the womb, but just before the baby is born, the angel that was teaching him taps him on the mouth and causes him to forget all the Torah that he learned (so that he will have the reward for re-learning it).
Because of the loss of Torah, there is a seven day mourning period (shiva)and therefore, the bris milah takes place the next possible day, the eighth day.
