Why: having enjoyed that vast heavenly world, with merit exhausted, they return to this mortal world, thus, following the righteousness prescribed by the three Vedas, desiring objects of desire, they attain only that which comes and goes, again emphasizing the temporary, cyclical nature of desire-driven pursuit.

In life: chasing pleasant experiences purely for the sake of the pleasant experience itself tends to require constant repetition, since the experience alone doesn't produce anything lasting on its own.

Teaching: notice which of your regular pursuits are purely for a repeatable pleasant experience versus which ones are building something more lasting, and make sure you're not investing everything in only the former.