Mainly, Poseidon hates Odysseus for blinding Polyphemus, who is Poseidon's son. Other reasons include their support for opposing sides in the Trojan war, Poseidon siding with the Trojans and Odysseus with the Greeks. The relationship between Odysseus and Athena may have also fueled Poseidon's anger, as Poseidon is rivals with Athena.
