From The Responsa of the RaShBa
18) Your Question: Why do we not utter berachoth (blessings) over all the mitzvoth (Biblical commandments)- for example, over such mitzvoth as lending a fellow-Jew some money, loading a camel together with a fellow-Jew, and so on?
My Response: This is a profound question already discussed by the Rishonim (medieval commentators). Many have found themselves entangled in the questions of why we utter berachoth over some mitzvoth but not over all, and over which we do utter berachoth and over which we do not. The answer does not depend on a single criterion for which I could formulate a rule. For one, no beracha is uttered over a mitzva, the observance of which does not involve performing a deed, such as the cancelation of debts in a shemitta (Sabbatical year) year, and so on. Similarly, no beracha is uttered over a mitzva, the performance of which depends on more than a single person, for if the second participant does not agree, the deed itself is cancelled, like the cases of gifts for the poor, and loans for the needy, and giving sums to charity and so on. This applies, too, to mitzvoth where the matter is sub judice, e.g., in matters of compromise where the decision of the judge still has to be accepted by those who stand before him. This is true, too, whenever the opportunity to observe a particular mitzva has come about as the result of first committing an "avera (a Biblical transgression), such as when returning stolen property or returning interest on a loan, for we were not commanded to rob in order to be able to return property, nor were we told to take the mother bird so as to be able to return her to her nest and set her free, and so on. Similarly, no beracha is recited over the giving of mat'noth kehunna, (priestly gifts), for the Jew is not giving something that belongs to him, but rather they are receiving something that belongs to the Almighty, that He has commanded us to give to them. The giving of a shekel a year and similar items are to be treated similarly, as we find (in I Chronicles 29): "For everything is of You, and it is from Your hand that we have given You something".
However, with regard to the redeeming of one's firstborn son or to the setting aside of halla and tithes, a beracha is recited, because one is not reciting the beracha over the giving itself, but rather over the setting-aside or the redeeming process, both of which depend on the person performing them only.
Furthermore, no beracha is uttered over the performance of a mitzva which is not really a mitzva at all, such as halitsa and yibbum (levirate marriage), which is originally for purposes of conception, but the woman is not commanded to conceive. This is also true of divorce, because there are cases where the divorce comes about because of an "avera, such as when her husband has not found anything wrong with her behaviour, but he divorces her because of his dislike of her. Now, even though some divorces have an element of mitzva in them, like with those women who were married in committing an "avera, and so it is a mitzva to divorce them. Our sages all feel it to be improper that the husband utter a beracha in some divorce cases, but not in others.
Moreover, one does not recite a beracha at an execution, for the Almighty takes pity on his creatures and does not desire the death of the evil, and so for this reason no beracha is uttered at such an occasion. This is reminiscent of the ruling that hallel (festive praise) is not recited on Rosh HaShana (the New Year) because Rosh HaShana is the Day of Judgment, and as we have learned that a Torah reader does not stop in the middle of the curses because one does not utter a beracha over violence and trouble. This applies, too, to the mitzvoth of visiting the ill, comforting the bereaved, and restoring peaceful relations between two Jews for these depend on others for their completion (a mourner may refuse to be comforted, and thus uproot the mitzva by not contributing to its completion).
Now, where do we learn that one is not to recite a beracha upon per-forming a mitzva which itself came into being as the result of an "avera? R. Eliezer ben Yaakov learned that if one has stolen a se'ah (measure of weight) of wheat, grinds it into flour and bakes it, and then recites the bracha and eats it, - i.e., if one pronounces a beracha over the bread when eating it, one is cursing HaShem (from Psalms 10). Rava explains the reasoning of R. Eliezer ben Yaakov's learning as the eating of the bread being a mitzva which has come into being as a result of an "avera. And where do we learn that any mitzva which may be uprooted, even though in a particular case it is not uprooted, is considered similar to an uprooted mitzva? It is learned in Chapter Ellu N'aroth, p. 39 (Talmud Ketubot), how it is that one is compelled to maintain that which he had previously scorned - even if she limps or is partially blind, unless she is not fit to have intercourse with him, when he is not allowed to remain her husband, for it is found in the Torah (Deuteronomy 22): "and she will be his wife" - a wife worthy of him.
In the Gemara we find: when a positive injunction comes, an "aseh (positive Biblical commandment), a negative injunction, a lo ta"aseh (negative Biblical commandment), is set aside. He said to him: where do we learn that "an "aseh comes and sets a lo ta"aseh aside? - the case of circumcision and leprosy, where it is impossible not to perform the positive injunction, but if this were not said, we would not need it for who would think of performing the "aseh? But now, since the "aseh can be uprooted, even though in this case it is not uprooted, it is considered similar to one which can be, which is not determined at all, since it is because of his own enjoyment that the Torah instructs him to marry. And since he said: "I don't need that enjoyment", the positive injunction in it has been uprooted.
From this we learn, with regard to the fear and respect one owes his father and mother, of one's obligation to stand before one's rabbi, that since the positive injunction can be uprooted one can be forgiven, since if a rabbi wills his dignity overlooked, his dignity is indeed overlooked. And where do we learn that no beracha is recited over mattenoth kehunna? (priestly gifts) - we consider the kohanim to have benefited from property possessed by HaShem, who then gave it to the Kohanim. When we see in Bava Kama that a person giving mattenoth kehunna is not giving them of his own possessions, but it is rather the Torah that says (BaMidbar 18): "and as for me, it is I that have appointed you over My Teruma", and this is the way Rabbi Joseph ibn Pelet, of blessed memory, replied to Rabbi Abraham ben Rabbi David.
Translated by R. Yoel Lerner