Freedom.
Owning better material items is only meant to facilitate freedom. I'm not a collector and never will be, even if I inherit two billion bucks tomorrow.
Examples:
- I'll get an expensive bike so me and my wife can go on an impulsive picnic in the outskirts of the city.
- I'll get an extra house in rural France so we're free to go there if we need the scenery for a few weeks (but will otherwise allow a local family to take care of it).
- I'll just invent stuff in my shed -- or compose the programs I always wanted to write on my computers -- if I'm never worried about money in the future.
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Rich people getting bored and even unhappy is just lack of imagination, intellectual and physical laziness.
Being rich and unhappy is entirely on them. It can be much better in every way but they're too busy complaining about non-issues to see it.