Teshuva

"Why is no berachah / blessing recited before performing the mitzvah of teshuvah / repentance? R' Menachem Simcha Katz shlita (Brooklyn, N.Y.) offers an anthology of answers, including the ones below. (Note that the parenthetical objections to some of the answers are from the cited work.)
(1) No berachah is recited because it is not within man's ability to complete the mitzvah, as only G-d can decide whether one's repentance will be accepted. For the same reason, no berachah is recited when giving charity, as the completion of the mitzvah is dependent on finding a worthy recipient. (One can argue, however, that this reason is not valid because we are guaranteed that heartfelt teshuvah will be accepted.)
(2) No berachah is recited because teshuvah is a mitzvah that comes about via a sin. Likewise, no berachah is recited over the mitzvah of returning a stolen object.
(3) No berachah is recited because the mitzvah of teshuvah is performed primarily in one's heart. Likewise, there is no blessing for bittul chametz / nullifying chametz.
(4) The formula for the berachah on mitzvot is, "Who has sanctified us with His commandments and has commanded us to . . ." However, teshuvah is not a mitzvah that one should perform because he was commanded to do so. Rather, it should be performed because one is self-motivated to repent. Indeed, it would be an insult to G-d for a person to say, "I am repenting because You told me to."
(5) No berachah is recited because repentance is not complete unless G-d can testify that the penitent truly intends to never repeat his sin. This level is very difficult to attain, and, in effect, any berachah recited may be a blessing in vain. For the same reason, no berachah is recited on the mitzvah of honoring parents, since honoring parents to the full extent of the law is nearly impossible.
(6) No berachah is recited because teshuvah takes a long time.
(7) No berachah is recited because teshuvah often occurs spontaneously."
(Simcha L'Ish Ch.38)
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