Because he wants to see what it would be like to become a Samana, he tells Govinda he is leaving. Govinda doesn't really agree with him. He also doesn't think that his father with agree with Siddhartha's decision either. Siddhartha asked his father's permission to leave his house to be on his own and join the ascetics. His father had a lot of displeasure in his heart and did not answer. Siddhartha stood in the same place for hours with his arms folded. His father saw that wasn't moving and that he would leave until he got a yes. So he let Siddhartha go become a Samana. He also told him before he left that if he learns anything new about enlightenment to come back and teach him.