Why does ebb and flow occur?

Because of the strong gravity, the Earth attracts the moon to itself, forcing it to revolve around it. In turn, the Moon also attracts the Earth, although not strongly, but sufficiently enough to attract the surface of the oceans. All of this causes the ebb and flow.
The ebb and flow represents the rise and fall of water levels in the world’s oceans. The reason for this phenomenon is the gravitational forces of the Moon and the Sun. Moreover, the Moon occupies a leading position due to the shorter distance to the Earth than the Sun. Ocean water on the side of the Earth that is facing the Sun or the Moon is 6500 km closer to them, and the opposite one is the same further, which makes the water attract with different strengths. The ocean rotates with the earth, so there is high tide twice a day and low tide twice a day.