Here's why:

* Erosion: V-shaped valleys are typically carved out by rivers and streams. The flowing water wears away the rock and soil, creating a steep, narrow valley with a V-shaped cross-section. The steeper the slope, the faster the water flows, leading to more erosion.

* Deposition: Deposition is the process of adding material to a landform. While deposition can occur in valleys, it doesn't form the characteristic V-shape.

In summary, V-shaped valleys are a product of erosion by rivers or streams over time.