Nachmanides on Genesis 12:2

And you shall be a blessing: You will be a blessing in that they will be blessed through you by saying, "God should make you like Avraham." And He also added that all the families of the earth would be blessed through him, and not just the people of his land. Or "and they shall bless themselves by you” [means] that they would be blessed on his account. And behold, this section did not elucidate the whole topic. As what is the reason that the Holy One, blessed be He, should say to him, "Leave your land and I will do good to you, like there had never been [before]," without prefacing that Avraham was a servant of God or perfectly righteous - or that He say a reason for his leaving the land, such that there would be in his going to another land, a coming close to God. And the custom of Scripture is to to say, "Walk in front of Me and listen to My voice and I will do good to you," as with David and Shlomo; and like the matter in all of the Torah: "If you will walk in my statutes" (Leviticus 26:3); "if you will truly listen to the voice of the Lord, your God" (Deuteronomy 28:1); and with Yitzchak, it stated, "on account of Avraham, my servant" (Genesis 26:24). But to promise him [blessing] because of his leaving the land is not a reason. Rather, the reason is because the people of Ur Kasdim did bad things to him due to his faith in the Holy One, blessed be He; and [so] he fled to the land of Canaan, but delayed in Charan - [such] that He told him to leave [Charan] as well, and to do what he thought [to do] at first. This is so that he serve Him and call the people to the name of God in the chosen land. And there his name would grow and all of the nations there would be blessed - not like what they did to him in Ur Kadim, where they would scorn and curse him, and they put him in a pit or into a fiery furnace. And [so] He said to him that He would bless those that bless him; and if an individual would curse him, [the latter] would be cursed. And this is the reason of this section, but the Torah does not want to write at length about the opinions of idolaters and to explain the issue that was between him and the Chaldeans regarding faith; just like it was brief about the matter of the generation of Enosh, and their rationale for the idolatry that they developed.