Once upon a time there lived a Sage namely Shilada. He was unhappy because he didn’t have any child.
So, Sage Shilada did penance to God Shiva for begetting a child.
Shiva impressed with Shilada’s penance gave Darshan and granted him a child, Nandi.
According to few Scriptures, it is believed that Nandi emerged as the outcome of a yagna, which Shilada performed.
Nandi was born with an armour and he was highly knowledgeable.
One day, Lord Indra visits Shilada’s ashram. Nandi goes and takes Indra’s blessings.
Meanwhile Indra informs Shilada that, his son Nandi has a short life span of only 8 years.
Hearing this Shilada cries uncontrollably, and Nandi overhears all the conversation, and leaves home to do rigorous penance to God Shiva.
Many months of rigorous penance by Nandi, pleases Mahadev and he gives Darshan.
Nandi says “ Hey Shiva, I can’t see my father unhappy because of my short life span, so please grant me long life and I want to do service to you, upto my last breath. Please allow me to be with you, on Mount Kailash “.
Shiva pleased with Nandi puts a garland of bells around Nandi. And thus Nandi became half human and half bull, and according to his wish became Shiva’s mount.
Being a Shiva’s devotee, I’m guessing the following for why Nandi born as a human, became mount to Shiva.
Nandi is shown as half bull and half human, this portrays the greatness of Shiva as Shankara and as well as Pashupathi nath, God and saviour of all animals.
Any devotee would be blessed to become Shiva’s mount and stay with Him forever on Kailash.
Shiva not only considers Nandi as gate keeper of Kailash, his mount, but also as His best friend and head of Ganas too.
The entire Universe resides in Shiva, and Nandi is so powerful and great to carry the Mahadev on him.
When your devotion is great and true, then even though you are born as an animal or human, Shiva gives equal love and opportunities to every living being and thus Nandi became Mahadev’s mount.