According to the Gemara, Rav Dusta'i b'Rebbi Yanai, the holy Tana, was being counted upon by Rebbi Acha b'Rebbi Yoshi'ah to put his life on the line -- presumably since a Devar Mitzvah was involved. In such situations, when the Tana'im tried to excuse themselves from the task, rather than saying "I am not worthy of having a miracle worked on my behalf," they would say, "Even if you think I am worthy of a miracle, are you so sure of my safety that you could ask my father (who loves me so) to agree to send me on such a mission?" We find a similar reaction on the part of Rebbi Shimon bar Yochai in Meilah 17b.

Along the lines of Derush, though, we could suggest the following. Rav Achai bar Yoshiyah was such an extraordinary Tzadik that even after death he "lived on" -- quite literally, according to the Gemara -- in the grave (Shabbos 152b). Rav Dusta'i could not argue that fulfillment of his mission might result in his death, since Rav Achai would not accept that argument. He would counter that, first, even if he died, it would be for Kidush Hash-m (by giving up his life for the Mitzvah involved) and receive unimaginable reward, and second, as a Tzadik he would live on in the grave in either case.

Rav Dusta'i therefore argued that the reward for Kibud Av va'Em is life in this world . Midah k'Neged Midah for serving one's parent's, the parents live a long life and the child is blessed to be able to continue to serve them (and to have his children serve him for long years). Therefore he replied, "I've honored my father so sincerely until now; I ought to be blessed to live longer in this world so that I may continue to serve him. If I die, who will replace my devotion towards him."

Hope that helps. Best wishes,

-Mordecai Kornfeld