The ancient Mesopotamians worshipped hundreds of gods. They worshipped them every day. Each god had a job to do. Each city had its own special god to watch over the city. Each profession had a god to watch over the people who worked in that profession like builders and fishermen.
To the Sumerians, each person had a god of their own, who looked after them. Their own special god talked to other gods on their behalf. Their personal god received a great deal of their worship time and attention. But no one god was more important than another.
The Babylonians and Assyrians believed in nearly all the Sumerian gods, plus more gods that each added. Unlike the ancient Sumerians, they believed some gods were more powerful than others, gods like the god of the sky, the sun, the air, and the crops. To the Babylonians, Marduk was the most powerful god. To the Assyrians, Ashur was the most powerful god.
The Sumerians, and later on, the Babylonians (southern Mesopotamia) and the Assyrians (northern Mesopotamia) all believed that everything good and bad that happened to them was the result of their gods pleasure or displeasure. They spent a great deal of time trying to make their gods happy. Their gods were not often happy. That's why Mesopotamian gods are sometimes referred to as the gloomy gods of ancient Mesopotamia.