When Abhimanyu, the valiant son of Arjuna, died in the Mahabharata war, Arjuna was overcome with grief. He decided to stop the war. Lord Krsna tried His best to convince Arjuna to stop grieving unnecessarily but it was all in vain. Finally, in order to show the impermanence of all worldly relationships, He took Arjuna to heaven. There Arjuna saw Abhimanyu enjoying the pleasures of heaven. Arjuna ran to Abhimanyu and embraced him. Abhimanyu was surprised and asked Arjuna “Who are you?” Arjuna told him that he was Abhimanyu’s father. Abhimanyu replied that he had had millions of births and in exactly which birth was Arjuna his father? Clearly Abhimanyu remembered nothing about his past birth and relationships. Arjuna saw the futility of attachments in this world. He had seen the proof that all worldly bonds are unreal.
Taken from this site.