And you should declare in front of Hashem, I removed the sanctified (ma’asros) from the house...and I did not forget. [Devorim 26:13].
Rashi explains that by saying “I did not forget”, the person who is making the declaration of biur ma’asros is saying: “I did not forget to bless You for [the mitzva] of separating ma’asros”.
Citing the Ri Magash, Shem MiShmuel explains that the “blessing” that this verse is referring to is the blessing of Shehecheyanu - thanking Hashem for the joy that he has in the heart - i.e., simcha shel mitzva - rejoicing over the performance of a mitzva. Shem MiShmuel teaches further, that in reality, not only is simcha shel mitzva a Torah requirement, it is the essence of each mitzva!
Interestingly, that regarding simcha shel mitzva the person who is making the declaration says “I did not forget,” (he does not say “I had simcha shel mitzva.”).
It appears that here, the Torah is teaching us that after doing a mitzva, the only “excuse” that one can give for not having had simcha shel mitzva during its performance is “I just simply forgot.” But if before performing a mitzva, a person remembered that he needs to have simcha shel mitzva, he has no excuse not to have it, because, as we saw from the teaching of Shem MiShmuel cited above, simcha shel mitzva is the heart of each mitzva.
