Answer:
Guru Nanak became guru because, he believed in one god and that Muslims and Hindus should not be separated and should be united instead. He went missing for 3 days at the age of 30 and was found that he was on the lake talking to god. God gave him the role in life to be a guru. His first words when he came out of the river were: 'god is neither Muslim or hindu' and that is how guru Nanak became guru.