The most likely reason why an ancient Greek might worship more than one god is that?

Ancient Greek religion and culture were based on Ancient Mesopotamia culture and the culture from the people that lived near the Crete Island.

As these two cultures gave a lot of importance on magical powers and nature's spirits, the Ancient Greek religion took a lot of these aspects. They were polytheistic they believe in many gods.

Consequently, Ancient Greek civilization wanted to give an explanation to all the natural phenomena around them so they created many gods and goddesses which personified each natural phenomena they want to understand. For instance, Poseidon was responsible for earthquakes and every event that happened in the sea.

They also created legends and myths in which the gods and goddesses appear and interact with humans. These myths or legends many times featured values, heroic events, among others which provide examples of how the humans should behave.