Guru Nanak is considered a holy leader because he was the founder of Sikhism, a religion that emphasizes the importance of equality, compassion, and devotion to God. He preached a message of love, unity, and social justice, and his teachings continue to inspire millions of people worldwide. He is revered as a spiritual guide and his teachings are followed by Sikhs as a way to attain spiritual enlightenment and lead a righteous life.