Here's why:

* Erosion: Young rivers have a lot of energy and are focused on cutting down through the landscape. This is called downcutting.

* Vertical Erosion: The primary form of erosion in young rivers is vertical, meaning they carve downward, creating a steep, narrow valley with steep sides.

* Narrow Channel: The river channel itself is narrow and often has rapids and waterfalls, further highlighting the V-shape.

As the river ages and reaches a more mature stage, the valley will widen and become less steep, eventually forming a wider, flatter valley with a more meandering river channel.