Fulfilling a commandment joins a person who fulfilled the mitzva to the Holy One, blessed be He. But this connection is not visible in the body of the one who performs the commandment. The advantage of the commandment of circumcision is that the change it effects is visible and felt in the physical flesh.
This is what the verse alludes to: “And My covenant shall be in your flesh as an everlasting covenant” (Genesis 17:13). The covenant with the Holy One, blessed be He, is imprinted and sealed in the physical body, to the point that it is recognizable even to the nations of the world.
(the Rebbe)
Circumcision with Joy
It was taught, Rabbi Shimon ben Gamliel says: Every commandment that they accepted upon themselves with joy, such as circumcision, as it is written, “I rejoice over Your word as one who finds great spoil” (Psalms 119:162), they still perform with joy.
(Shabbat 130a)