and there was strife between the herdsmen of Abram's livestock and the herdsmen of Lot's livestock.
Mo' moolah, mo' problems. The more you have, the more you need to keep what you already have. As Abram and Lot both grew rich, they also grew apart. There wasn't room enough for them to both be rich in the same place at the same time in the same way. The land could not sustain them both and it produced hostility between those who were in charge of caring for the flocks.