Why are Athena and Poseidon rivals?
There are many reasons why the two deities don’t see eye to eye. Throughout several stories, the two are constantly undercutting each other, and offering their favors to the enemies of the other. And as far as I know, one of the key incidents behind the rivalry was their dispute over who would be the patron God of Athens.
You see, Greece used to be a collection of city-states, rather than a unified nation. And each city chose one God as their patron. The patron of a city would receive the greatest amount of worship in that city, so being selected as the patron of a major city like Athens was something the gods of Olympus coveted.
Both Athena and Poseidon wanted the position. Rather than fight over it, the two decided to settle the matters with a competition of gifts. They would each offer the city a gift to be judged by the city’s citizens and leaders, and the god that provided the best gift would become the city’s patron.
Poseidon offered a gift first. He brought down his triton and created a well of salt water that flowed from the city to the ocean. That created a canal that gave Athens access to the Mediterranean Sea.
Access to the sea is a big deal. It allows a city to trade much more efficiently, and it makes the movement of troops in and out of the city much easier. Poseidon thought this gift would secure him an easy win.
Except… well, Athena is a warrior goddess, but she’s also the goddess of wisdom. Instead of anything fancy, she simply touched the floor with her spear and created a little tree. That turned out to be the first olive tree, and olive oil would go on to be considered one of Athens most valuable resources. Plenty of cities had ports, but only Athens had olives.
This gift secured Athena the win, and the city of Athens ended up named after her. Poseidon Greek god supposedly never got over this defeat, which either sparked or cemented the rivalry. You can find out more about Poseidon here: https://mythologysource.com/poseidon-greek-god/.