Why did Duryodhana tell Arjuna to ask for a boon?

Why did Duryodhana tell Arjuna to ask for a boon? Before the Kurukshetra war, when Pandavas were living in a forest in exile, Duryodhana placed his camp just opposite to the pond where Pandavas were living. But when Duryodhana was taking a bath, the heavenly Prince Gandharvas came there and they got into the fight. Duryodhana was defeated and captured in this fight and it was Arjuna who set him free. After this, Duryodhana felt very ashamed and told Arjuna to ask for a boon and Arjuna told him that he will definitely ask for a boon but only when needed.