Some interesting stories here. The reality is if we did not regret selling/trading/losing books, there would be no hobby. The value in any collectible is based on nostalgia. If everyone had kept every favorite book or never regretted parted parting ways with them, there would be a lot more supply and much slower increases in value, driven only by new collectors joining the hobby.
Anyway, in my case, I acquired a CGC 9.4 Marvel Spotlight #5 from my dad, who had purchased it off the newsstand. I sold it for $900 because I needed the money for college. It's one book I've always wanted to reacquire. Been waiting to find and win one at auction in same or higher condition and it always just kept creeping up beyond my available funds at the time. Now, I'd probably need to sacrifice another book or two to fund getting one, but never say never.