Why is the metacentre required to be above the center of gravity for small-angle stability?

In naval architecture, the metacentre ($M$) must be positioned above the centre of gravity ($G$) to ensure a vessel maintains static stability.

When a ship is at rest, the centre of buoyancy and the centre of gravity are vertically aligned. When the vessel tilts, the centre of buoyancy shifts because the shape of the displaced water changes. If the metacentre—the point where the vertical line through the new centre of buoyancy intersects the ship's midplane—is above the centre of gravity, the interaction between the forces of weight and buoyancy creates a righting moment that restores the ship to its upright position.12

Conversely, if the metacentre is below the centre of gravity, these forces create a capsizing moment that increases the angle of heel, rendering the vessel unstable. The distance between these two points, known as the metacentric height, serves as a critical index of stability for small-angle inclinations.123