Since an important part of fulfilling the mitzvot is having the right intention while doing so, how come we do brit milah on boys when they are infants when they have no say in the matter (in fact, if we asked them, they’d probably refuse)? Wouldn’t it be better for men to fulfill the mitzvah when they are at least bar mitzvah so they could fulfill the mitzvah willingly?
As you know, the reason we do circumcision is because G-d commanded Avraham to perform this mitzvah and perpetuate it among his descendents. The command from G-d to Avraham is two-fold: He simultaneously commands Avraham to circumcise himself, while also commanding him to do so to the male members of his household. Regarding his son Yitzchak and his progeny, G-d tells Avraham that they must be circumcised specifically on the eighth day of life.
From here we learn that a Jewish father is required to circumcise his Jewish son as an infant on the eighth day; but if he hasn’t done so for whatever reason, he is required to do so before the child comes of age, from which time the person himself becomes responsible to do so.
Now, as you note, intention is extremely important. That’s why, according to ancient teachings, although Avraham intuited the mitzvot before they were given, and actually preformed them, he refrained from becoming circumcised until he was commanded. Since it’s something that’s only done once, he preferred to do so with the right intention, the foremost of which is intending to fulfill G-d’s will.
However, the mitzvah as commanded by G-d to Avraham regarding the descendents of Yitzchak was that a Jewish father is commanded to do brit mila on his Jewish son. That’s the father’s mitzvah, for which he is required to have intention. This may be compared to other obligations a Jewish father has regarding his children, like supporting them and teaching them Torah, even though these are obligations that the child will assume upon coming of age.
Your suggestion that if we “asked” the infant he would most probably say “No” is an interesting point, but not necessarily true. So you ask, “Who would willingly undergo circumcision?” Thousands of Jews from the former Soviet Union where brit mila was forbidden, but are now free to be Jewish, are the answer to that question. If the infant was really able to understand, his answer would also be “Yes”. This covenant is an integral expression of his being Jewish – if not from day one, at least from day eight.
That being said, even if one was circumcised as an infant, there is still a way to fulfill the mitzvah as an adult. Our Sages connect the following verse to brit mila: “I rejoice over Your statute as one who finds great spoils” (Psalms 119:162). The Chatam Sofer notes a contradiction in the wording: “finding” implies happening upon something with no effort of one’s own; “spoils” are something one obtains through the efforts of battle. He explains that the former refers to an adult who “finds” himself already circumcised since childhood: While the mitzvah has been performed on him, he’s done nothing to perform the mitzvah himself. The verse therefore enjoins him now as an adult to battle the drive to abuse the brit by upholding and maintaining its sanctity. In this way, rather than circumventing it, he confirms his commitment to the covenant.