KATHOPNISHAD ADHYAK 1 CHAPTER 1 ,SHLOKA- 29

                                                 यस्मिन्निदं विचिकित्सन्ति मृत्यो

                                                यत्साम्परायेमहति ब्रूहि नस्तत् ।

                                                    योऽयं वरोगूढमनुप्रविष्टो
               
                                                 नान्यं तस्मान्नचिकेता वृणीते॥ २९

                             YASMIN IDAM VICHIKITSANTI MRYUTYO

                            YAT SAMPARAYE MAHATI BRUHI NASTAT|

                             O"YAM VARO GUDHAM ANUPRAVISHTO

                        NA ANYAM TASMAN NACHIKETA VRUNITE ||29

                    O DEATH -YAMA TELL ME IN WHICH MEN HAVE DOUBT AND

                  WHICH IS ABOUT GREAT PASSING -DEATH-WHICH IS MYSTERIOUS

                            ,NACHIKETA WANTS THAT BOON NOTHING ELSE  ||29

This is the last of the first chapter.So after the exchange between Yama  and Nachiket, Nachiket is telling that he wants only one boon and nothing else. This almost like when we bargain in stores or buying car, The sells man offers the price  and we offer different and he comes with different offer  and then after some exchange we give ultimatum. We offer so called Final price  and if not accepted , walk out. This is similar to that.
    Yam has offered different things to Nachiket instead of the Knowledge about Life after death. and now Nachiket is telling that he does not want anything else but the one that he asked for. The life after death is a topic of discussion in every religion. Many religion believe that we will see GOD after death only . Many have no idea of reincarnation. Many have ideas of ghosts  and heaven  and hell  and Saitan. So in short what happens after death is mysterious  and so all of us want to know  and who is better to tell us than the God of Death - YAMA.