The greater fool theory states that the price of an object is determined not by its intrinsic value, but rather by irrational beliefs and expectations of market participants.
The bottom line however, no matter what sort of investing/trading you wish to do, the most important thing is your risk management. I’ve been in this game for over 26 years. I’ve seen so many people “blow up” because when the market was hot, and they were making gains every day, that they thought they were geniuses. But, then the market decided to roll over into a bear market, or even just a good correction, and they got crushed, losing it all. It’s all about risk.