Hindsight is 20/20.
Everyone has situations where they missed out on something massive (cue teenager emoting at the word "massive") for whatever reason.
Maybe you sold all your nVidia in 2018
Maybe you had a chance to invest in a company that you thought was stupid that later would have made you wealthy and retired
Maybe you sold NFLX after the Qwikster debacle
For me, it's very topical and upsetting. In late 2020 I turned down an offer to join a little known (at the time, to mainstream America at least) big data, software and services company called Palantir.
The company at the time had a market cap of $40bn @ $20 a share and I had an offer with some meaningful equity.
I thought their compensation package was low (and it was, compared to the spiking covid market we will all remember) and I thought they were too much a Fed & Academic / R&D play (they weren't, though it looked a lot like that at the time).
5 years later, will be a $400bn valuation @ $156 a share.
Millions of dollars never realized because I was "smart".
Funny because on a long enough timeline, "smart" and "coward with no vision" are almost the same thing.