What about the legacy you leave after yourself? Be it invention, great article or a book, children who are better than you, or great businesses or non-profits.
If everybody thought about "just ending in a coffin and everything is pointless", humanity would've ceased existence long time ago.
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My best time investment was investing in my discipline by training to wake up at the same time in the morning and doing morning exercise on alarm.
My most horrible time waster was watching Netflix. Since the past few months, I've canceled it, and now I have much more time to invest in myself.