Fuck You money is an old concept on Wall St, and I think it is worth thinking about deeply because it tells you a lot about who you are and what you want.
Fuck You money is an old concept in general, 200 years ago you would have been called a man of independent means, and you would have either spent your time playing cards, being an amateur academic or politician.
Some people think $50,000 a year will finance a lifestyle they can be happy with for the rest of their lives - which works out to $65,000 pre-tax for some. They then assume they can get 5% for Treasuries, which means they need $1.3mm.
This math of course ignores the effect of inflation and also of constant withdrawals on a volatile endowment. Those Treasuries are going to be whipping your $1.3mm around, and when they are at a MTM low, your fixed withdrawals are going to be taking a huge bite on the principal, thereby lowering future returns by a lot. Same reason why defined benefit plans aren't very viable.
Also, most people cannot help but spend up to their means (or beyond). When you start making more, you will start spending more, and that $50,000 will start to look unrealistic. It will climb to $100,000 and then $200,000 and then higher. A lot of people scoff at this and say they can never spend $200,000 a year. But if you think about the ratio of how much you spent in your first working year out of college, versus how much you spend in your tenth working year out of college, you can see how easy it is for cost of living to rise.
There is simply no limit to how much a person can spend before they say "it is enough". Lots of poorly disciplined sports stars plow through millions with little to show for it.
It takes a lot of discipline to spend well beneath your means even when you are rich.
These are the reasons why I think Fuck You money is a key concept for a professional trader to grasp.