Here's why:

* River Erosion: V-shaped valleys are primarily formed by the erosive power of rivers. As a river flows downhill, it cuts into the surrounding rock and soil, gradually carving out a deep, narrow channel.

* Downcutting: The river's erosive force is strongest at its base, causing it to cut downward into the land. This downward erosion is called downcutting.

* V-shape: The combination of downcutting and the river's natural tendency to meander creates the characteristic V-shape of the valley, with steep sides and a narrow bottom.

Important Note: V-shaped valleys are typically found in areas with young mountains or where the land is still actively being uplifted. As the valley ages, the river may widen its channel, and the valley will evolve into a wider, more open shape.