Brachos 61b: Rabbi Akiva replied, “All my life I agonized over the verse, ‘…and you shall love Hashem…with all your life.’ It means, even if they should take your life from you. I pondered, ‘When will this come to me so I can fulfill it? Shall I not rejoice?'”
It says he wanted and waited for this opportunity all his life and now, at the moment the Romans were torturing him to death, he was joyful it was happening to him.