Shaivite scholars state that "the original reason for associating the bull with this god was simple, the great masculine potency which each embodies, and the rage and physical power of which both are capable."
As the creature on which Shiva rides, Nandi embodies the permanent inner strength which can be acquired by controlling physical strength and violence (the name means: he who grants joy). Only those who have conquered desire and achieved self-knowledge can ride the bull like Shiva himself.
Jansen, Eva Rudy. The Book of Hindu Imagery, The Gods and their Symbols: Holland, Binkey Kok Publishers, 1998.
Chatterjee, Gautam. Sacred Hindu Symbols: New Delhi, Abhinav Publications, 2001.