Sarah's dislike for Hagar primarily stemmed from feelings of jealousy and resentment when Hagar bore Abraham's son, Ishmael, while Sarah remained childless for a significant time. In the biblical narrative, particularly in Genesis 16, Sarah encourages Abraham to have a child through Hagar, her maid, because she felt unable to bear children. However, once Hagar becomes pregnant, she starts to treat Sarah with contempt, which exacerbates Sarah's feelings of inadequacy and dissatisfaction.
The other points mentioned, such as Ishmael being stronger or Abraham's favoritism, are interpretations that may arise from various readings of the texts, but the primary source of conflict lies in Sarah's complex emotions regarding her status and motherhood in relation to Hagar and Ishmael. Ultimately, the situation leads to tension and conflict that results in Sarah's mistreatment of Hagar.