Love trumps fear Yevamot 4 and 5

 What do we do when there's a conflict between observing either a positive commandment or a negative commandment? We have the general halakhic principle “a positive mitzva comes and overrides a prohibition -אָתֵי עֲשֵׂה וְדָחֵי לֹא תַעֲשֶׂה.” Ramban explains why a positive commandment takes precedence over the negative commandment. The underlying motivation of observing the positive commandment is love of God. The underlying motivation of observing the negative commandment is fear of God. Ramban teaches us that love trumps fear.

TB Yevamot dappim 4-5 searches for the source of this general principle that a positive commandment comes and overrides a negative one. The sages learn this principle from the juxtaposition (semukhin-סְמוּכִים) of two verses. “As it is written: “You shall not wear diverse kinds of wool and linen together. You shall make for yourself twisted fringes on the four corners of your covering with which you cover yourself” (Deuteronomy 22:11–12). These verses teach that despite the prohibition against wearing diverse kinds of wool and linen, it is permitted to prepare ritual fringes of diverse kinds, e.g., sky-blue dyed threads of wool on linen garments. This shows that the positive mitzva of ritual fringes overrides the prohibition of diverse kinds.”

After analyzing many different possible proofs and rejecting them for one reason or another, this proof of shatnes and tzitzit is accepted as the correct source. In the course of the discussion we learn that the book of Deuteronomy is fertile ground for proofs of semukhim. Even though Rabbi Yehuda generally does not accept the principle of semukhim; nevertheless, the book of Deuteronomy is different.

"§ And from where do we derive that Rabbi Yehuda does derive homiletic interpretations in Deuteronomy? As we learned in a mishna: A person may wed a woman raped by his father and one seduced by his father, despite the fact that his father’s wife is forbidden to him. Similarly, he may marry a woman raped by his son and one seduced by his son. Although one is prohibited by Torah law from marrying the wife of his father or the wife of his son, these prohibitions do not apply to a woman raped or seduced by them. And Rabbi Yehuda prohibits him from marrying a woman raped by his father and a woman seduced by his father.

“And Rav Giddel said that Rav said: What is the reason for Rabbi Yehuda’s opinion? As it is written: “A man shall not take his father’s wife, and shall not uncover his father’s skirt” (Deuteronomy 23:1). The latter expression: “And shall not uncover his father’s skirt,” is referring to a skirt that has been seen by his father, i.e., any woman who has had relations with his father may not be uncovered by his son, meaning that his son may not marry her.

“And from where is it known that the verse is written with regard to a woman raped by his father? It is from the previous verse, which deals with the halakhot of rape, as it is written: “And the man who lay with her must give the maiden’s father fifty shekels of silver” (Deuteronomy 22:29), and juxtaposed to it is the verse: “A man shall not take his father’s wife and shall not uncover his father’s skirt.” This shows that Rabbi Yehuda does derive homiletic interpretations from juxtaposed verses in Deuteronomy." (Sefaria.org translation)

The Gemara gives two reasons why Deuteronomy is such fertile ground. "The Gemara asks: But as Rabbi Yehuda does not generally derive homiletic interpretations from juxtaposed verses, what is the reason that he derives these interpretations in Deuteronomy? The Gemara responds: If you wish, say that it is because it is evident (mokakh-מוֹכַח ) from the context; and if you wish, say instead that it is because this verse is extraneous and is therefore free for this inference. (mufnai-מוּפְנֵי ) "