I was reading about the Aztec god Xolotl and how he was associated with fire, lightning, and death. He was also, apparently, associated with Venus. If this is true, is there any particular reason why the Aztecs associated this particular god with the planet Venus?
Xolotl and Quetzalcoatl
Xolotl was the twin brother of Quetzalcoatl, and where both associated with the twin phases of Venus as evening and morning star.
In their roles as evening and morning star both gods constitute inimically paired phases of Venus, with Quetzalcoatl acting as morning star was the harbinger of the Sun rising or "re-birthing". His twin Xolotl acting as the evening star and harbinger of the Sun setting or "dying".
Cycle of life and death
In this way both gods and their associated cycles of rebirth and dying complement and complete each other. They divide the life/death process of cyclical transformation into its two inimically partnered constitutive phases with the one leading for birth to death and the other from death to birth. Because of this collaboration they can accomplish what they could not on their own, the completion and re-initiation of the Fifth Sun's life/death cycle.
Xolotl does Quetzalcoatl's bidding as his inimic partner in Mictlan, just as Quetzalcoatl does Xolotl's bidding as his inimic partner in the sky.
Xolotl helps the Sun return to life from its journey in the region of the dead