The Reason for the Mitzva
טעם למצווה
A person to whom a firstborn son is born must redeem him after thirty days by giving the kohen the value of five shekels by the sacred shekel, as it is stated (Shemot 13:2): "Sanctify to Me every firstborn, whatever opens the womb among the Children of Israel, of man and of beast; it is Mine." And it is stated (ibid. 13:13): "And every firstborn of man among your sons you shall redeem." And it is stated (Bamidbar 18:16): "And those to be redeemed, from a month old you shall redeem, according to your valuation, five shekels of silver by the sacred shekel, which is twenty gera."
To understand the reason for the mitzva of pidyon bekhorot (redemption of the firstborn), we must first note that in everything primary and primordial, the hand of God is more revealed. It is true that whoever lifts his eyes to see and ask who created these things understands that everything is from God; but the routine in which we see the world again and again with all its external manifestations makes us forget the primordial questions, such as: where did everything begin, and how does everything live? But when a person encounters for the first time something new that has just been created, such as a baby, his heart is stirred, and he draws closer to faith. So the Maharal of Prague explains (Gevurot Hashem ch. 38), that everything primary resembles to some degree the Creator, blessed be He, Who is "the beginning of existence," and therefore there is holiness in firstborns. There is holiness in the first fruits that grow anew each year on the tree, with which we must fulfill the mitzva of bikkurim and bring them up to Jerusalem to the kohanim. Likewise, there is holiness in the first son born, to the point that he cannot live an ordinary, mundane life in this world without our redeeming him as the Torah commands.
Another important aspect should be emphasized in the reason for the mitzva of pidyon bekhorot. Precisely the holiness and special virtue revealed in the firstborn son may create a complication both for the parents and for their son. The parents may become excessively proud of their firstborn son, to the point that they think as if they themselves created their son, and forget that they are messengers and partners in bringing their son's soul down into the world. In addition, parents tend to pin great and exaggerated hopes on their firstborn son, which may foster negative traits in him, such as arrogance, a tendency toward selfishness and an exaggerated fear of failure. Against this pride of the parents, which is later projected onto their son, the Torah comes and obligates the father to redeem his son, in order to emphasize: do not think that this son is your creation and that he is yours. In truth he is holy and belongs to God, and in order for him to be able to live a mundane life in this world, you must redeem him.
A father who merits the mitzva of pidyon bekhorot has a great privilege, because this is a rare mitzva that does not come to every person. First, because anyone to whom a daughter was born first is exempt from pidyon bekhorot, since only a firstborn son must be redeemed and not a daughter.
Kohanim and Levi'im too, and even Israelites who married the daughter of a kohen or the daughter of a Levi, are exempt from redeeming their firstborn son (as explained below in section 9).
Likewise, when the first pregnancy ended in a miscarriage, the son born after it will be exempt from pidyon bekhorot. And if the firstborn son was born by cesarean section, he is not redeemed. It turns out, then, that fewer than a third of all parents merit to fulfill the mitzva of pidyon bekhorot. And since it is a rare mitzva, people make an effort to fulfill it with enhancement: they invite many guests and hold a large meal, with meat and delicacies.