“Now Abraham journeyed from there toward the land of the Negev, and settled between Kadesh and Shur; then he sojourned in Gerar.” Gen. 20:1

It is interesting that Abraham heads to Gerar, as thus far Abraham has avoided cities. Rashi suggested Abraham moved from where he was because of what happened with Lot and his daughters, assuming the matter became well-known. But Kadesh and Shur were large cities, and Gerar was large enough to have its own king.

Rabbi Sforno says Abraham moved here so that he could spread the word of God.  Josephus wrote that Abraham excelled at spreading the word that God was the one true God and he was prepared to debate about it to convince others.

Perhaps having seen the fate of Sodom and Gomorrah, Abraham became less concerned with quiet country life and more concerned with ensuring the large cities had enough righteous people in them to spare them from destruction, and hoped he could be a positive influence to some in the cities.