Genesis 13:7

And contentions arose between the shepherds of Abram's flock, and the shepherds of the flocks of Lot; for the shepherds of Abram had been instructed by him not to go among the Kenaanaee and the Pherizaee, who, as yet, had power in the land, and to restrain the cattle that they should make no depredation in going to the place of their pasture: but the shepherds of Lot would go and feed in the grounds of the Kenaanaee and Pherizaee who yet dwelt in the land.

AND THERE WAS A QUARREL. Rashi wrote, “Because Lot’s shepherds grazed their cattle in other people’s fields, Abram’s shepherds rebuked them for this act of robbery, but they replied, ‘The land has been given to Abram, and he has no [son as an] heir, and so Lot will be his heir. Hence this is not robbery.’ Scripture however states, And the Canaanite and the Perrizite abode then in the land, so that Abram was not yet the legitimate owner.” This is a Midrash of our Rabbis.
But I wonder: The gift of the Land declared to Abram was for his children, as it is said above, Unto thy seed will I give this land, so how can Lot inherit it? Perhaps the shepherds heard of the gift and they mistook its meaning, for Scripture states that in the meantime, the land belonged neither to Abram nor to Lot. Accordingly, the verse stating at the outset, for their possessions were great, intended to say that because of their extensive possessions, the land could not support them, and Lot’s shepherds therefore found it necessary to bring their cattle into fields that had owners. This was the cause of the quarrel.
By way of the plain meaning of Scripture the quarrel concerned the pasture as the land could not support them both. When Abram’s cattle were grazing in the pasture, Lot’s shepherds would come into their territory and graze their cattle there. Now Abram and Lot were both strangers and sojourners in the land. Abram, therefore, feared that the Canaanite and the Perrizite, who inhabit the land, might hear of the abundance of their cattle, [whose great number was made apparent when Lot’s shepherds encroached on Abram’s land, thereby combining the flocks], and drive them out of the land or slay them by sword and take their cattle and wealth since the mastery of the land belonged to them, not to Abram. This is the purport of the verse, And the Canaanite and the Perrizite. Scripture thus mentioned that there were many peoples dwelling in that land, they and their cattle being innumerable, and the land could not support them and Abram and Lot.
From the word oz (then) — [And the Canaanite and the Perrizite abode ‘then’ in the land] — it appears to me that the nations dwelling in the land at that time were those who live in tents and have cattle, some of them converging on one district and grazing there for a year or two and then journeying from there to another district in which they had not previously pastured. And so they continued to do, as is customary among “the children of the east.” The Canaanite and the Perrizite were thus “then” in the land of the south, and in the following years the Jebusite and the Amorite would come there.

ויהי ריב בין רועי מקנה אברם ובין רועי מקנה לוט, there was an argument between who would succeed to drive out the other from the available grazing land they would find.

והכנעני והפריזי אז יושב בארץ. This is why a quarrel between two brothers who lived side by side was especially embarrassing and harmful to the image of Avram among the surrounding tribes. When those tribes would see the brothers quarrel, they would assume that neither of them was a peace-loving individual and they would reason that they had even more reason to quarrel with either one of those recent immigrants.

ויהי ריב, we already explained the meaning of the words והכנעני אז בארץ on 12,6. In this verse the tribe פריזי has been mentioned which had not been featured previously. This tribe also resided in the region of Beyt El and Ai.