I don't look back on it that fondly nowadays, but it would probably be Mr Money Mustache's classic "The Simple Math of Early Retirement". I am not on a path to FIRE really, but in my early twenties it was really incredibly eye-opening. At that age, I had very much assumed that the world had to work according to "work hard for 45 years, get the biggest house you can, buy as much stuff as you can, on your 65th birthday…
Of course, don't spend on frivolous things. Do you really need the designer bag, instead of just a bag? Do you need that road-trip with your friends in your 20s? Do you really need to impress that boy/girl by taking them to a michelin restaurant for a date?
OTOH what is the opportunity, and life cost of being frugal or too frugal? Perhaps the designer bag caught the attention of the boss, and 'earned' their respect, which ultimately translated to a promotion. Perhaps you ended up finding your life parrter in one of your road-trip friends. Perhaps you didn't get the boy/girl because you cheaped out on the date. Etc.
Life is to be lived. Some moderation is required. Not too much though.