Surface tension forces the surface of water to curve due to its tendency to reduce the surface area of water.
The surface tension of a liquid is in itself a tangential force acting parallel to the surface of the liquid due to the cohesive force of attraction between the molecules of the liquid.
The highly cohesive nature of water molecules means that the molecules at the air/water interface are more stuck together, creating elastic-like surface tension on the water. Thus, the water surface curves inward forming a concave shape.
In other words, the cohesive force of water molecules causes surface tension which in turn causes the surface area of water to reduce or curve inward.
More about surface tension can be found here: https://brainly.com/question/11348644
