Parshat Lech Lecha is the first Parsha of the Torah that speaks about the Jewish people. Avram and his family were the first Jews. Our people first began when Hashem said to Avram, לך לך, "leave the place where you are now living." This call to Avram represented what Hashem wished for the Jewish people, namely, that our people be apart from the other nations of the world, and that we develop our own unique way of life and traditions. Although the Torah was not yet given, the values of the Jewish people were already established by our ancestors. Avram and Sarai took it upon themselves to influence as many people as they could to believe in the one true Hashem. The people that came under the influence of Avram and Sarai accompanied them on their journeys.
One of the people that stayed with Avram and Sarai was Lot, the nephew of Avram. Avram traveled to Canaan twice. The first journey was the original one when Hashem said to Avram לך לך מארצך וממולדתך ומבית אביך, "go forth from your country, from the place of your birth, and from your father's house." The second journey was when Avram returned from Egypt (where he went initially because of the famine in Canaan). It was during the return from Egypt that the separation of Avram and Lot began. This separation is worth examining more carefully.
The Torah tells us that Avram and Lot were very wealthy and had a lot of animals when they left Egypt. They were so wealthy that they could not live together because the land could not sustain both of them. This led to arguments between Avram's and Lot's shepherds. Since Avram was a רודף שלום, a man of peace, he preferred that he and Lot separate so that there would be no conflicts. Lot chose to go to the ערי ככר הירדן - the cities of the Jordan Valley, where the people were wicked, while Avram settled in the land of Canaan.
But, if we look at the Torah more closely, we will see that there were deeper reasons for why Avram and Lot were separated. A careful look into the Pesukim which tell us of Avram's journey to Canaan, reveals hints that Avram and Lot were already becoming more distant from each other. The Torah tells of Avram's first journey to Canaan in when it says ויקח אברם את שרי אשתו ואת לוט בן אחיו ואת כל רכושם אשר רכשו ואת הנפש אשר עשו בחרן, "And Avram took Sarai, his wife, Lot, his nephew, and all of their belongings" (בראשית יב:ה). Notice the order that the Torah uses. It first mentions Sarai, then Lot, and then all of their possessions. When the Torah speaks of Avram's return to Egypt, the Pasuk says ויעל אברם ממצרים הוא ואשתו וכל אשר לוולוט עמו הנגבה, "and Avram went up to the Negev along with his wife, all of his belongings, and Lot was with him" (שם יג:א). The order here is different from the one described earlier. Here, Sarai is mentioned first, then their belongings, and then Lot. Why was Lot mentioned before their possessions in פרק יב but after the possessions in פרק יג? The change in the order is not accidental. When the Torah makes even the slightest change, it is carrying a lesson for all of us. In the first Pasuk, Lot is written in the middle before the mentioning of their possessions in order to show that he was part of Avram and Sarai's household. He went where they went and did what they did as if he were their son. By the time we get to the Pasuk at the beginning of פרק יג, Lot is no longer as close to Avram and Sarai as he used to be. That is why the Pasuk says ולוט עמו, as if to say that Lot was going with them but was no longer part of them. That is also why the Torah in the later chapter mentions Avram's and Lot's possessions separately. Lot no longer wanted to be like a son to Avram and Sarai. He was becoming his own person. Unfortunately, the type of person that he was becoming was not as good as Avram. This is evident in the fact that Lot was willing to go to fertile Sodom, despite the fact that the people were wicked. Lot was willing to expose himself and his family to the bad surroundings and to the corrupt people in order to be able to make more money.
The Pasuk says that there was a quarrel between the shepherds of Avram and the shepherds of Lot. What were they arguing about? The Midrash explains that they quarreled about what the proper conduct was in regard to their sheep. When Avram's shepherds would take their animals to pasture, they muzzled them so they could not eat from someone else's property. The shepherds of Lot did not do the same with Lot's animals. The shepherds of Avram asked Lot's shepherds why they were not muzzling their sheep. Their reply was that you should tell us why you are muzzling Avram's animals. After all, Avram's property will eventually be inherited by Lot since Avram has no children. Therefore, you should allow the animals to eat as much as possible so they will be in good shape when Lot inherits them.
With this story, the Midrash teaches the following lesson: כשאדם צדיק אף בני ביתו צדיקים כמותו, (פסיקתא רבתי פרשה ג ד"ה ד"א ביום השמיני) when a man is righteous the members of his household are also righteous. However, וכשאדם רשע אף בני ביתו רשעים כמותו, when a man is wicked the members of his household are also wicked. The shepherds that worked for Lot learned from him and were acting like him. The shepherds of Avram had already adopted practices of the צדיק that they were working for. It was obvious, therefore, that they could not live together. It was for this reason that Avram told Lot that they would have to separate from each other. Lot had already made decisions about the direction that his own life would take and this direction was incompatible with the way of life of Avram, Sarai and their followers.