No Lies

The Abarbanel asks why we find the concept of “vidui” by the mitzvah of maaser, wherein one must attest that he fulfilled the mitzvah, while we don’t find such a thing in regards to any other mitzvah.

He answers that no individual Kohen or Levi is able to demand that the masser be given to him, as the individual can choose to give it to whichever Kohen or Levi he wants. Therefore, whenever one of them requests it, he can send them away by saying he gave it to someone else. For this reason, the Torah commands the person to testify that he did, in fact, give the maaser, in order to teach him not to lie and to distribute it properly.