Katha-1-2-2-śreyaśca

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

If both the sweet and the good can be pursued at will, why do the mankind, in general, pursue the sweet? This is explained; it is true that they can both be pursued; still, as they are not easily distinguishable by persons of poor intelligence, either in respect of the means to their attainment, or in respect of their fruits, both the sweet and the good become as it were mingled, and approach man. Therefore the intelligent man examines both the sweet and the good as a flamingo separates milk and water and having considered in his mind their relative weight, divides them both and follows the good alone, as preferable to the sweet; but the man of poor intelligence, incapable of such discernment, pursues the sweet, such as cattle, sons and the rest, for the purpose of fattening and preserving his body, etc.

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