ומל ה אלקיך אֶת־לְבָבְךָ֖ וְאֶת־לְבַ֣ב זַרְעֶ֑ךָ לְאַהֲבָ֞ה אֶת ה אלקיך בְּכׇל־לְבָבְךָ֥ וּבְכׇל־נַפְשְׁךָ֖ לְמַ֥עַן חַיֶּֽיךָ
The Or HaChaim asks: since the previous pesukim already describe Klal Yisrael doing teshuvah, why is there a need for the milah of the heart? Why does the possuk say "ומל... לבבך" and not "עָרְלַת לְבַבְכֶם" like it says in Parshas Ekev (10:16), when presumably it is the orlas halev that needs to be removed?
Why does the pasuk add that Hashem will also circumcise the hearts of one's children?
Why does the pasuk end with "לְמַ֥עַן חַיֶּֽיךָ", which would seem to imply loving Hashem for one's own self-serving purposes?
The possuk in Ekev and the possuk in Nitzavim are referring to two different types of milah of the heart. The possuk in Ekev which describes a person removing their own ערלת הלב in referring to a person needing to perfect their own heart from its desires to be open to fully serving Hashem. It refers to a person achieving internal perfection. The possuk in Nitzavim describes a person led astray because of what they saw done around them, ותראו את שקוציהם וכו. Even after doing teshuva and returning to Hashem, there is always a lingering concern that external influences will lead them astray once more. Because it is beyond a person's human capability to completely eliminate external pressure, Hashem says I will מל your heart - cut off the connection between the eyes and the heart so that external sights no longer influence the person's internal world.
This act of disconnection automatically leads to the love of Hashem being implanted within one's family forever, as there will no longer be external temptations. In this context, "לְמַ֥עַן חַיֶּֽיךָ" means that this act of milahwill allow one's entire life to be completely consumed with the love of Hashem (based on Likutei Sichos, vol. 29.)
The Netziv explains למען חייך doesn't mean to ensure one's physical life but rather a fulfillment of one's spiritual potential. The Neztiv writes in a few places that חיrefers to spiritual vitality and fulfillment hence צדיקים קרויין חיים וכו. He adds here that love requires reciprocity and outside Eretz Yisrael where there is not a direct connection with Hashem, one can not feel the same level of fulfillment, of חיות and that is the beracha the possuk is giving that Hashem will give a person the ability to feel the absolute fulfillment of spiritual life.
