Water seeking its own level is a consequence of pressure depending on depth. In a bent U-tube full of water, the water in one side of the tube tends to push water up the other side until the pressures at the same depth in each tube are equal. If the water levels were not the same, there would be more pressure at a given level in the fuller tube, which would move the water until the levels were equal.
Work Step by Step
Liquid pressure equals weight density times depth. A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid.