Here's why:

* Lithosphere: This layer is composed of the crust and the uppermost part of the mantle. It's relatively cool and strong, making it brittle and susceptible to breaking under stress.

* Asthenosphere: The layer beneath the lithosphere is the asthenosphere. It's hotter and more fluid, allowing tectonic plates to move and interact.

How faults form:

* Tectonic Plate Movement: The lithosphere is broken into large plates that move and interact. This movement creates stress on the lithosphere, causing it to break along planes called faults.

* Stress and Strain: When rocks are subjected to stress (forces acting on them), they deform. If the stress exceeds the rock's strength, it will break, creating a fault.

Therefore, faults primarily form within the lithosphere due to its brittle nature and the stresses caused by tectonic plate movement.