Why devotees chant Har Har Mahadev for Lord Shiva

While offering prayers to Lord Shiva, devotees chant Har Har Mahadev. Har Har Mahadev have religious meaning that depicts the thought of Lord Shiva he wants to spread to his Bhakts. Lord Shiva is the first guru of universe and chanting Har Har Mahadev gives you the serene feeling.