Why are there tides on Earth but not just one constant water level?

Tides are caused mainly by the Moon’s gravitational pull, with the Sun contributing as well. Because gravity is stronger on the near side of Earth than the far side, water is stretched into bulges on both sides of the planet. As Earth rotates, coastlines move through these bulges, producing high and low tides. The Sun can make tides stronger or weaker depending on whether it lines up with the Moon or sits at right angles to it.