Before destroying Sedom, God "consults," as it were, with Avraham. The Torah lets us in on God's "thought process" when deciding to divulge this information to Avraham: "For I know him, that he will instruct his children and his posterity to keep the way of God by doing what is just and right…" (18:19). Rashi explains the expression "yedativ" ("I know him") in this context as affection. Meaning, God professes His special love for Avraham specifically on account of his commitment to teaching the ways of God to his offspring.
One explanation for this basis of God's love for Avraham likens his instruction to his children to that of a skilled worker. A practitioner who works only for his wages will not necessarily encourage his children to pursue the same occupation. He will rather advise them to seek other professions that make a better profit for less work. One who truly loves what he does and views it as bearing paramount significance will strongly urge his children to follow his example is career selection.
God's love for Avraham thus resulted from Avraham's love for God, as reflected by his commitment to disseminating the ideals of Godliness throughout the world. As the Chatam Sofer notes, others before Avraham - most notably, Chanokh - had also achieved exalted spiritual heights. However, true devotion to the word of God entails more; it results in a relentless effort to share it with others, to teach, assist, guide, and inspire. Avraham was chosen to father God's nation, then, because he possessed the spark necessary to ignite the eternal transmission of the "mesora" (tradition). Only one with such an inflamed devotion to the way of God can implant these ideals within the hearts of his progeny.