Dipper was never accepted among his people. People always told him it was because he wasn't manly enough, or because of his birthmark. But he knew the real reason that he was shunned: He couldn't sing. And if he can't sing then he is useless as a siren. So he left. Had no plan where to go except just leave. Maybe just hide out in a cave and wait for his life to expire. Until a certain pirate captain finds him and changes everything.