Otto I, the son of Henry the Fowler and first German king to be crowned Holy Roman Emperor, was crowned Holy Roman Emperor by Pope John XII in 962 because he defended the Romans from an attack from their enemies in Italy lead by Berengar of Ivrea. After his defeat against Berengar, Otto imprisoned him. Not long after, the Pope felt uneasy about Otto's dominance and started to maneuver against him. Also, in 955 Otto I destroyed the Magyars, which were a threat to the Holy Roman Empire because they were Pagans and had the ability to convert everyone to their religion as apposed to Christianity. Here is a picture of Otto I, also known as Otto the Great:
This is the crown of the Emperor of the Holy Roman Empire. Many Emperors after Otto I called themselves Emperors of the Holy Roman Empire because they were Otto I's successors. The Popes were infuriated because the other emperors didn't deserve the crown as Otto I did. Pope John XII gave Otto I the crown because he saved the Holy Roman Empire from the Pagan Magyars in 955. The other emperors didn't do anything as generous and heroic to earn such a powerful crown, but being the selfish, power-hungry emperors they were, they snatched the crown when they had the opportunity.