Neonatal circumcision does not utilize anesthesia (it is assumed less than 40% of the procedures use any pain blockers) so the infant feels the full brunt of the procedure. Hyper-sensitivity in newborns is well known now and the stress and pain observed during circumcision are well documented and studied. Shock occurs, hear palpitations, blood loss, and at times death (it's been recorded on average 100-200 newborn infants die in the US annually due to the complications during circumcision. Complications that did not have to occur).
from the article: Europe's shudder at circumcision