Margen
To a €50 note, everything is the same: it doesn't know whether it came from your salary, a prize or a tax refund. To your brain, it isn't. Richard Thaler won the Nobel Prize in 2017 for describing, among other things, this mental accounting: we label money and spend it according to the label.
Salary is sacred, a bonus is 'for fun', a tax refund feels like manna from heaven — even though it was yours all along. At the casino, winnings are 'house money' and get gambled away cheerfully.
The classic experiment: plenty of people will cross town to save €5 on a €15 item, but wouldn't do it to save the same €5 on something costing €125. Identical discount; different mental drawer.
A euro has no memory. We give it one — and it costs us.