Garen was Taric's lifelong friend. His nobility status allowed him to keep Taric safe from being accused of high treason and executed.
But even Garen (who still believed in Taric but deep down thought he was wasting his talents by fooling around) turned on Taric after Voidborn killed the Shield of Valoran's men, resulting in his banishment and sentence to the Crown of Stone by the Might of Demacia himself.
Taric appreciates the beauty in all things (music, life)
This, his role as Aspect of the Protector, and especially past experiences, have made him an enemy of the Void.