Why was guru Nanak chosen to be a guru?

Guru Nanak wasn't chosen to be a Guru he became a Guru by travelling around India and other countries. You see he was brought up as a Hindu but didnt believe in the whole religion. anyway when he was older he had married and had 2 sons, as well as this he worked in an office and learnt about many other different religions. When he was 30yrs old he had a bathe in the river and he disappeared. his family thought he were died, so were very sad. However 3 days later he returned and said ''GOD TOOK ME AND TOLD I SHOULD TEACH PEOPLE THE TRUTH'' so he travelled around many different countries teaching people what they believe is the truth. And it became known as Sikhism. so as the finder of Sikhism he just became arandom guru!!! hpe this helped!!! xxx