There was a book released by the main writer for Pokemon (Takeshi Shudo) back in 1999 called "Pocket Monsters: The Animation". Basically, there's a chapter where Satoshi (Ash) is five-years-old, and his mom is tucking him in bed, and reminessing about his father/her ex-husband.
Satoshi's mom, Hanako (Delia?), and her mom, owned an inn in Masara (Pallet) Town where wandering trainers would come and rest. One day, a wanderer, a star trainer, came in, and grew fond of Hanako, and she grew fond of him. At the time, they were both eighteen. Anyways, eventually, they got married, and she got pregnant with Satoshi. When Satoshi was born, he left her.
Hanako then explained to Satoshi that his grandfather (his father's father) was also a star trainer, and Satoshi then fell asleep. She then said to herself that she knows one day Satoshi would leave her, because it's in his blood -- and he did, five years later.