Why did Hapy Egyptian god flood the Nile River?

In ancient times, people tended to have little understanding of science, and they did not know how to explain the changing seasons or a lack of rain or extremes of weather. They responded to the world around them by creating stories to try to explain why certain things were happening. The worship of an item or an object (including a river) means that thing was considered very important. In India, for example, the sacred river Ganges was considered like a divine mother, capable of protecting Hindu devotees and spiritually cleansing them from sinful activities. Similarly, in ancient Egypt, the Nile was considered like a god because without water, life would be impossible: there would be no commerce, no farming, no irrigation of crops, and no travel by boat.

Early human beings often believed rivers (and mountains too) contained a spirit, or were actually gods and goddesses in disguise; so they worshiped these special places, in hopes the spirit or deity living there would treat them kindly. The more important a place was in the culture, the more likely it was to be treated as holy. (This belief that even inanimate objects possessed a soul or spirit and should be worshiped is called "animism.") Thus, ancient Egyptians did not just see a river when they saw the Nile. They saw something that kept them alive and sustained them, and they worshiped the spirit they believed was protecting and nourishing them.