Adam Clarke on Genesis 13:6

Their substance was great - As their families increased, it was necessary their flocks should increase also, as from those flocks they derived their clothing, food, and drink. Many also were offered in sacrifice to God.

They could not dwell together - 1. Because their flocks were great. 2. Because the Canaanites and the Perizzites had already occupied a considerable part of the land. 3. Because there appears to have been envy between the herdsmen of Abram and Lot. To prevent disputes among them, that might have ultimately disturbed the peace of the two families, it was necessary that a separation should take place.