Why is Nandi ji the vehicle of Lord Shiva?
Lord Shankar’s vehicle is Nandi ji i.e. an ox. The bull is a very hard working creature. He is calm and innocent despite being powerful.
In the same way, Lord Shiva, despite being supreme and powerful, is extremely calm and so naive that one of his names Bholenath is also famous in the world.
The way Lord Shankar had consumed Kamdev and conquered him, similarly his vehicle would not have failed. He has complete control over his work. Also, Nandi ji is also considered a symbol of purushartha.
Even after working hard, the bull never gets tired. He is constantly doing his deeds. This means that we too should always do our work. Just as Nandi is dear to Shiva due to his actions, we can also get the blessings of Lord Shankar.