Or HaChaim on Genesis 11:31/אור החיים על בראשית י״א:ל״א

1 ויקח תרח את אברם בנו. Terach took his son Abram, etc. We must understand this verse as an extension of a popular proverb quoted by Baba Metzia 75, that if someone experiences bad luck in one location and he does not move to another location he only has himself to blame if his bad luck continues. When Terach realised that his son Abram did not beget children in Ur Casdim he left in the direction of the land of Canaan to see if his luck would improve. The Torah shows us that Abraham was so important in the eyes of the other members of his father's household that they all joined in the migration in order to stay close to him. This is why the Torah adds the words ויצאו אתם, "they emigrated with them (Abraham and Sarah)."