Here's some cartoon responses to this oft asked question:

http://www.stopinfantcircumcision.org/toon.htm

Fathers are often strangely competitive with their sons. Sometimes it comes down to I don't want that little upstart to have anything I don't (or can't) have.

Also, sometimes it is a way of validating what happened to the adult. Many men have a difficult time internalizing that otherwise circumspect and loving parents would subject them to a harmful and unnecessary genital surgery. The parents may have had the child's best interests in mind, but the medical industry did NOT.

Fortunately this is changing in the U.S. As circumcision is now in the single digits in all other English speaking societies, can we be far behind?