The Ramban (13th-century Spain) poses the question thus:
"This section of the Torah has not adequately explained the issue.
Why should God tell Abraham, "Leave your land and I will extend to you the greatest good that has ever been", without first indicating to us that Abraham was a servant of God or a perfectly righteous individual?
Or, the text should state a reason for God's injunction to Abraham to leave his land, namely that he will achieve closeness to God in the new location. The convention of the Torah is to state, "Walk before Me, listen to My voice, and I will reward you"... This is the pattern throughout the Torah:
-"If you will follow My statutes..., then I will provide the rains in their due season." [Lev. 26:3];
-"If you will surely listen to My voice..., then [I] will raise you above all other nations of the earth." [Deut. 28:1]...
But to promise Abraham such reward solely on account of his leaving his land makes no sense."
Other commentators have argued that God chose Abraham because he was intelligent and obedient. He had reached knowledge of God on his own, without revelation, and did as told. That was the combination God needed.