Parshas Acharei Mos discusses the divine service done by the Kohen Gadol, and the Medrash tells us that the Kohen Gadol brings with him, into the holiest part of the mishkan, the merit of the mitzvah of Bris Milah. Why did he bring this particular mitzvah in with him on Yom Kippur?

     Explains the Munkaczer Rebbe, R' Chaim Elazar Spira zt"l, at every Bris Milah, the assembled guests recite כשם שנכנס לברית כן יכנס לתורה לחופה ולמעשים טובים - "just as he entered the covenant, so may he‬‎ enter into Torah, marriage and good deeds." The reason that we do not express a similar wish at a Bar Mitzvah, for instance, is that at the time of the Bris, the infant is doing the mitzvah in it's purest form, without any selfish or egotistical motives. Thus, we wish for him to enter into Torah and marriage with the same purity of heart.

     There is usually a subconscious element of self-interest, pride, or even arrogance that tends to enter our minds, with regard to the mitzvos we perform. This being the case, how can the Kohen Gadol bring such tainted mitzvos into the קודש הקדשים - holiest of holies, where the true intent of each mitzvah is revealed and the motive that inspired it is bared?

     It is, therefore, for this reason that the Kohen Gadol enters with the mitzvah of Bris Milah accompanying him, for this is the one true mitzvah which is undergone without even the slightest trace of selfishness.