Like human, Greek gods also have weaknesses, like Zeus. Zeus weakness is beautiful women. Zeus always wanted are beautiful women. He went down to earth just to have the women who he think was beautiful. Talking about beautiful women, it was much better to have not just a women but a goddess, like Hera. Greek consider Zeus to be the Supreme Ruler of the gods but when it comes to Hera, its like Zeus was under her.
Zeus love Hera so much. This is the reason of his weakness. When Hera wants something, she use this love to get what she want. In book 14, Hera was able to seduced Zeus and send him to sleep so that Zeus will not interfere her in helping the Achaians. While there asleep, Sleep said Poseidon to that Hera wants him to help the Achaians and that's what he did.
In book 15, Zeus woke up and saw his brother, Poseidon, helping the Danaans and Hector was injured. He got mad at Hera but Hera denied that it was her plan. She explained that maybe Poseidon just wanted to help because Achaians were being crushed and maybe he can just tell him to stop interfering. In an instance, Zeus' anger faded away and did the advice of Hera. He sent Iris to tell Poseidon to stop meddling in the war and sent Apollo to breathe courage to Hector again so that his plan will be follow.
Zeus love for Hera make him weak. He loved her so much that this love was used by Hera trick him.