Chasidut zu Rut 3:22

‎“in order to impose a prohibition upon his ‎person.”
From the wording of the Torah it seems clear ‎that permission to utter vows or oaths is granted only if the ‎person doing so did so in order to strengthen his ability to obey ‎certain commandments that he was in danger of violating had he ‎not reinforced his determination by means of a vow or oath. An ‎example of such a vow is found in the Book of Ruth 3,13, where ‎Boaz reinforces his undertaking to be Ruth’s redeemer if a closer ‎relative refuses to marry her, by adding the words: ‎חי ה'‏‎, “by the ‎Life of Hashem”. He did so in order to bolster his resistance ‎to the evil urge that might advise him not to enter into such an ‎‎(apparently) inappropriate marriage.‎

They established [reading of] Ruth on Shavu'ot, to say that, by means of B'nai Yisrael being chosen through the Giving of the Torah, they are instruments to draw near the converts, because B'nai Yisrael are called the First of His Crop which is the fruit for which everything was created. This is as it says (Gen. R.), "B'reishit -- for the sake of Israel which is called 'reishit.'" The meaning is that "reishit" is the beginning and the inner essence, as it says (Prov.), "The 'reishit' of wisdom is the awe of God." Therefore it is called the Day of Bikkurim, when the fruit is ripe. But B'nai Yisrael "are excepted from the generality in order to teach to the entire generality" (Sifra, principles of R' Yishma'el), as it will be in the future (Zeph. 3:9), "For then I will make the peoples pure of speech, etc. [so that they all invoke Adonai by name and serve Him with one accord]." And then (Eccl. 7:8) "The end of a matter is better than the beginning of it" [see Ruth 3:10]. And in truth, according as B'nai Yisrael elevate from the nations, they are themselves exalted even more. And this itself is the meaning of putting "na'aseh" before "nishma," for B'nai Yisrael understood this, that they were chosen in order to draw everyone near, and they said (Ex. 24:7), "Everything that Adonai has spoken we will do." It is written here "na'aseh" and it is written there (Gen. 1:26), "Na'aseh adam," as it says (Gen. 12:5) "the souls that they had made," which Onkelos translated "that they had subjugated to the Torah", and it is through this that "we will hear." As it is with the particular -- as the improvement [tikkun] of the deeds, in order to draw each physical deed close to the spirit so that the soul shines in the person -- so it is in general -- as the improvement [tikkun] of K'lal Yisrael to draw near from the nations, so that the Torah shines in them. It is written (Micah 7:20), "You will give truth to Ya'akov, hesed to Avraham," for B'nai Yisrael are in truth God's portion and His Torah, but it is also part of the aspect of Truth to do hesed, to bring near those who are far off and who come for the sake of Heaven: "truth to Ya'akov" is the Torah, "hesed to Avraham" is that we should draw close also from the descendants of Lot -- Ruth the Moabite....