Katha-1-2-1-anyacchreyo'nyadutaiva

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

Having thus tested the disciple and found him worthy of the knowledge, Death said ‘good is one thing and pleasant is another.’ Both these, the good and the pleasant, serving different ends, bind man competent for both, subject to the varying conditions of caste, orders of life, etc., i.e., all men are propelled in their mind by these two actions; for, according as one wishes for prosperity or immortality, he attempts at what is good and what is pleasant. Therefore as men have to perform acts to obtain what is good and what is pleasant, all men are said to be bound by these. These two, though connected with the realisation of one or other of the covetables of man, are opposed to each other, one being in the nature of knowledge and the other of ignorance. Thus, as both these are impossible to be pursued by the same individual without abandoning either, happiness falls to him who, of these two, rejects what is merely pleasant, being in the nature of ignorance, and pursues only the good. But he, who is not far-sighted, who is ignorant and who pursues only the pleasant, is separated from, i.e., misses the true and eternal end of man.

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