Many of the offerings described in Parshas Vayikra are completely voluntary in nature. If these mitzvot are so important, why isn't their performance obligatory, and if it they aren't, for what purpose did God give them?
The Steipler, in Birkas Peretz, explains that just as somebody who loves God will be inspired to go beyond the letter of the law in serving Him, so too somebody who doesn't yet feel a love of God but performs voluntary mitzvot can awaken within himself feelings of love of God. For this reason, God made certain mitzvot optional, because if they were obligatory, performing them would not be accompanied by the same sense of donating one's time and energy for the sake of God and would not produce the same result.
He adds that in this sense, converts and ba'alei teshuvah (returnees) have an advantage over those who grew up in religious families due to the fact that they view themselves as voluntarily accepting the yoke of all of the mitzvot, which creates within them unparalleled feelings of love for God.
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THE WRONG SUITCASE
The Darkei Mussar writes that of the thousands of parables developed by the legendary Dubno Maggid, there were three which the Kotzker Rebbe declared were said with Divine inspiration. One of those three was used to explain the verse from this week's Haftorah, Isaiah 43:22.
A businessman once returned home from his travels and hired one of the porters at the train station to carry his luggage to his home. Upon arriving at the man's house, the porter put down the bags and approached the man to receive his payment. The traveler took one look at the boy and informed him that he had mistakenly brought the wrong suitcases.
The surprised porter questioned how the businessman could make this claim with such certainty when he hadn't even seen the bags, which were still outside. The man explained that it was clear from the boy's appearance that he had sweated and exerted tremendous effort to transport the luggage. As the bags which belonged to the businessman were filled with lightweight items which wouldn't have required such exertion, it must be that the porter mistakenly brought the wrong suitcases.
Similarly, Isaiah related that God told the Jewish people, "v'lo oti karata Yaakov" - You haven't called Me in your performance of mitzvot. The Ohr HaChaim HaKadosh writes (Bamidbar 23:21) that the study of Torah and the performance of mitzvot should be enjoyable and invigorate a person. Isaiah teaches elsewhere (40:31), "v'kovei Hashem yachalifu koach" - Those who look to and trust in God will be constantly strengthened and refreshed. Just as the businessman informed the porter of his error, the prophet chastises the Jews that they must not be learning and doing mitzvot for God's sake. The proof of this claim is that instead of feeling renewed and energized, "ki yagata bi Yisrael" - You grew weary of Me.
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Why may offerings be brought from domesticated animals (Leviticus 1:2) but not from wild animals?
The Daas Z'keinim explains that it would have required significantly more effort to track down wild animals and capture them for use as offerings. Instead, God kindly commanded us to bring offerings from domesticated animals which are readily available to spare us unnecessary exertion.
Rabbeinu Bechaye cites the Talmud (Bava Kamma 93a) which teaches that it is better to be pursued than to be the one chasing others, and for this reason, doves are the only species of bird which may be brought as an offering, as they are the most pursued of all birds. Similarly, wild animals are disqualified because they typically pursue their prey, while domesticated animals are more likely to be pursued and may therefore be brought as offerings.
The Paneiach Raza cites Rashi (Genesis 1:22), who explains that God blessed the domesticated animals that they should be fruitful and multiply but not the wild animals because the latter category includes the serpent, which enticed Adam and Eve to sin. Because wild animals weren't included in God's blessing, they may not be brought as offerings.
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RETURNING THE STOLEN OBJECT
The Torah commands (Leviticus 5:23) a thief to return the item that he stole. If somebody stole an esrog and returned it after Sukkos ended, did he fulfill the mitzvah of returning the stolen object?
The Talmud (Bava Kamma 66a) rules that if somebody steals chometz before Pesach, he may fulfill his obligation to return the stolen object by giving it back to the owner after Pesach. Even though it originally had monetary value and now has none since it is forbidden to benefit from chometz which belonged to a Jew during Pesach, this type of damage isn't clearly recognizable, as the chometz still looks the same as when it was stolen, and the Torah doesn't hold the thief responsible for such indiscernible damage.
The Pri Megadim (OC 656) maintains that this law would also apply to a thief returning a stolen esrog after Sukkos, and by doing so, the thief would fulfill his mitzvah of returning the stolen object even though it is now almost worthless, since the damage to its value isn't readily apparent.
However, the Pischei Teshuvah (CM 363:1) differentiates between the two cases. He suggests that the thief may return the chometz after Pesach because it appears identical to other chometz which didn't belong to a Jew during Pesach and from which it is permissible to benefit. The only difference between the two types of chometz - the ability to use them - cannot readily be discerned by the naked eye. On the other hand, everybody knows that the value of an esrog decreases sharply after Sukkos, and this is considered a form of damage which is clearly recognizable, in which case the thief could not fulfill his obligation by returning the esrog, but would instead need to reimburse its owner for the monetary value that it had at the time of the theft.