Here's why:

* Increased Thermal Motion: As the temperature of a conductor increases, the atoms within the conductor vibrate more vigorously. This increased thermal motion makes it harder for electrons to flow freely through the material.

* Electron Scattering: The vibrating atoms act as obstacles, scattering the electrons and hindering their movement. This scattering effect leads to increased resistance.

Important Note: While this is generally true for most conductors, there are some exceptions. For example, certain semiconductors have a decreasing resistance with increasing temperature.