Admitting the ability to deny
וכי תזבחו זבח-תודה ליקוק לרצנכם תזבחו
When you bring a thanksgiving offering, you shall offer it willingly
When the Torah instructs us regarding bringing a thanksgiving offering, it tells us to offer it willingly. This is quite surprising, as shouldn’t this be true regarding all offerings? In fact, the Torah tells us in general to bring offerings willingly. Why then does the Torah specify this requirement with thanksgiving offerings?
A verse in Hallel which we recite says, “אנה יקוק כי אני עבדך אני עבדך בן אמתך פתחת למוסרי, O’ Hashem, I am your servant, I am your servant, the son of your maidservant. You have freed me from my restraints. לך אזבח זבח תודה ובשם יקוק אקרא, I will offer a thanksgiving offering to You, and I will call out in the name of Hashem” Some explain the verses as follows: We were born into servitude of Hashem, as He created us from nothing. He sustains us at all moments, and our ability to make any movement only comes from Him. Therefore, it would seem automatic that we couldn’t even lift a finger against Hashem, to do anything against His will. How could we do anything against His will, since our very ability to move comes from Him?!
However, this is one of the true wonders of Hashem. He Himself gives us the very strength we need to be able to go against Him. This is the only way for Him to reward us for observing His commandments. If we couldn’t really go against His will, how could we deserve reward? The verse which states, פתחת למוסרי, You have freed me from my restraints, is referencing this concept. I am completely bound by Hashem’s will, without the ability to turn left or right. My very ability to exist and move come from Hashem’s will. But You, Hashem, have released me from my bonds, by giving me the ability to go against You.
Therefore, the conclusion of the verse is that I will bring You a thanksgiving offering, calling out in Hashem’s name. The word thanksgiving, תודה, also means to admit. The concept of admission only exists if a person can deny something, and nevertheless admit it’s true. Once Hashem gives us the ability to go against His will, King David in the verse states that he will not take advantage of this ability. Instead, he will bring a thanksgiving offering, and call out in the name of Hashem. The proper path is to follow Hashem’s will, despite the ability to not.
We can perhaps apply this interpretation to the verse we started with. We are adjured to bring a thanksgiving offering willingly, despite this requirement being true for all offerings. Traditionally, a thanksgiving offering is brought by someone who had a miraculous salvation from death. They’ll praise Hashem for raising them from the pit of destruction. However, their praise and thanks isn’t simply for being given continued Earthly existence. Rather, it’s also for being granted the continued opportunity to follow Hashem’s mitzvos. How is there any significance to mitzvah observance, where it should be impossible to go against His will? That significance is given by Hashem Himself, Who gives us the very ability to go against His will. Thus, it makes sense that the verse should specify with the thanksgiving offering that it should be given willingly. The very ability to bring this offering willingly is worthy of thanks.
 Based on Teshuvos Oneg Yom Tov Introduction
 Leviticus 22:29. This is how the Oneg Yom Tov seems to translate לרצונכם. Cf. Targum Onkelos, Targum “Yonasan” and Rashi ad. loc., who understand it that this offering shall be for our desire. Meaning, Hashem should be satisfied with our handiwork, that we do it properly. Rashi also brings another explanation that the offering must be לדעתכם, meaning not מתעסק, which is worthy of its own discussion why does specify this here
 Leviticus 1:3
 Psalms 116:16,17
 The Oneg Yom Tov cites this explanation from the Maharit. I couldn’t find this exact explanation, but perhaps he intended the latter’s Tzafenas Paneach Emor Derush § 1 s.v. וכיוצא בזה היה אומר, where he writes in the name of his teacher’s uncle: “פתחת למוסרי ונתת בידי הבחירה והרצון אל כל אשר אחפוץ”
 See Nehemiah 9:6
 See Shulchan Aruch Choshen Mishpat § 87, where we see this idea applied to מודה במקצת
 See Rashi to Leviticus 7:12, based on Psalms Chapter 107. See Berachos 54b, which only connects this verse to Birkas HaGomel, and Rosh Berachos 9:3, who brings a dispute how connected the two concepts are