Why does Zeus help the trojans?

Zeus helps the Trojans in Homer's "Iliad" primarily due to his desire to maintain balance among the gods and fulfill his promise to Thetis, the mother of Achilles. He aims to ensure that the Trojans experience success in battle to counterbalance the Greeks’ earlier victories, as he is trying to honor the fates. Additionally, Zeus often acts based on personal relationships and divine favoritism, which influences his decisions in the ongoing conflict.