To understand Shantanu's story, listen to the story of Mahabhisha:
King Mahabhisha had earned so much merit that he ascended to Indra's heaven. There he danced with the apsaras to the music of the gandharvas and drank sura; the wish-granting tree Kalpataru gave him everything he desired.
One day a breeze blew the goddess Ganga's garment aside. The devas looked away, but Mahabhisha could not resist: he stared directly at her naked breasts.
Indra cursed Mahabhisha to return to earth, and he told Ganga to take human birth and break his heart.
Mahabhisha was reborn as Shantanu, king of Hastinapura.
Inspired by: Jaya: An Illustrated Retelling of the Mahabharataby Devdutt Pattanaik.
Notes: This story is from Chapter 8: Mahabhisha Becomes Shantanu.