Here's why:

* The relationship: The speed of a wave (v) is directly proportional to both its frequency (f) and wavelength (λ). This is represented by the equation: v = fλ

* Constant speed: For a given medium, the speed of a wave remains constant.

* Balancing the equation: If you increase the frequency (f), to keep the speed (v) constant, the wavelength (λ) must decrease proportionally.

In simpler terms: Imagine a wave like a vibrating string. If you make the string vibrate faster (double the frequency), you squeeze more waves into the same space, meaning each wave is shorter (half the wavelength).