Katha-1-2-10-jānāmyahaṃ

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

Delighted, Death says again: I know that the treasure, i.e., the reward of Karma, because it is sought after like a treasure, is not eternal; for, that which is constant, i.e., the treasure named Paramâtman cannot be reached by things not constant. That treasure alone, which is in the nature of uncertain happiness, can be obtained by uncertain things. Therefore, by me, though I know that the eternal cannot be attained by ephemeral aids, has been propitiated, the fire Nachikêtas leading to the attainment of heavenly joys with ephemeral things. By virtue of that, I have attained the position of authority, this office of Death known as Svarga eternal, but only relatively.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda