According to the Torah's inheritance laws, a firstborn son receives a double portion. This means that if there are three sons, the inheritance is divided into four portions, and the older son receives two. The Torah prohibits the father from transferring the firstborn’s right to another son.

Abarbanel states two reasons for these laws. The first reason is that all firsts are beloved by Hashem. This is also the reason for the mitzvah of bikurim (first fruits). To elaborate, all firsts allude to Hashem, Who is the quintessential first. Hashem therefore sanctified firstborns, and one of the elements of this sanctity is his right to a double inheritance (see also Baal HaTurim and Hagahos HaGra to Orach Chaim 480:1).

A second reason is that a person loves his children because they will continue after him, and he especially loves his firstborn son, whose birth initiated that continuation. It is therefore fitting for him to be distinguished from the brothers with a double portion (see also Taamei HaMitzvos by Rav Menachem HaBavli §1).

Ateres Moshe (Ki Savo §13) writes that the double portion comes from the brothers' portions. They have to give the firstborn an extra portion because of their obligation to honor their oldest brother.

Alternatively, we may suggest that the double portion of the firstborn son stems from the fact that while a father may have many sons who continue after him, his position can only be filled by one person, and the oldest son is most eligible for this. For example, kingship passes from father to firstborn son. Even when the father does not have a position that may be filled by the firstborn son, the firstborn son generally fills the role of family leader in his father’s absence. He therefore deserves a portion like all the brothers, plus an additional portion as the continuation of the father (see also Otzar Ephraim to Bamidbar 1:5). By giving him fully double of that received by any of the other brothers, the father distinguishes him clearly as the family leader after his passing. The role of the firstborn as the father's successor is essential to the family's continued stability (Rav Hirsch). We may further suggest that the Torah forbids transferring this right to another son out of the concern that this transfer will not be accepted by the firstborn, and this would spark a feud after the father’s passing.

In Parashas Vayechi, Yaakov transferred the firstborn portion from Reuven to Yosef. Several commentators (Daas Zekeinim,Rosh, and Sforno) explain that this was an exception to the rule because Reuven sinned in the incident of Bilhah and became unworthy of the firstborn right (see Bava Basra 133b). Yaakov transferred it to Yosef, who provided for him in Egypt.