Nachiketa was either smart as he did not ask the boons in reverse order of what he had actually asked or he made spiritual progress very fast seeing through the true nature of things (so we were told). As first boon, which itself is an intelligent concatenation of multiple wishes, he made sure he will go back to his father alive who would be pacified and also recognize him. As second boon, he wanted to know the method to attain heaven to enjoy happiness. Yama graciously gives two extra boons here: naming this method by the name Nachiketa besides granting a garland to him.
Now the third boon struck Yama as a thunderbolt as Nachiketa innocently asks what happens after death. Yama tempts Nachiketa with other objects that any ordinary person would love to possess: wealth, power, beautiful women or men (as the case maybe), long life etc. It is unclear why Nachiketa would fall for these as he had already covered most of these through his second boon. Yama was unwilling to lose control over Nachiketa (just as he had control over all the mortals) by revealing this knowledge to him. It could also be seen as Yama testing Nachiketa of how serious he was for this knowledge and what price he was willing to pay. Much to the dismay of Yama (or happiness if he was testing), Nachiketa views all these “enjoyable” things as trash, thereby establishing his credentials as the rightful recipient of this knowledge.
Yama then proceeds to teach Nachiketa what the real nature of man is, elucidating methods to realize it and what happens after its realization. Swami Yogatmanandaji, in his class in Rochester retreat, explained each verse in detail and at the end of a particular verse that mentions Atman as the cause of life-forces in a person, he anticipated questions galore, but unsurprisingly the plethora of questions that followed were about rebirth introduced in the subsequent verses. For some reason, Swamiji was in a combative mood with logicians and he used rank-bad examples with illogical conclusions to show “drawbacks” of logic itself amidst peals of laughter from the audience. However, later Swamiji addressed the issue of drawback of logic in a “logical” way stating that logic is built upon our mental framework and is therefore, limited by it.
The Upanishad concludes, like a happy ending in any story, with Nachiketa attaining immortality by knowing the true Self and it exhorts us to achieve the same by following this path. Thus rendering Yama ineffective, Nachiketa became an embodiment of death to the Death itself! In the words of the Upanishad, Etad vai etat – (To Yama) this (Nachiketa) is verily that (Yama or death)!