I just bought this for $100. I now declare it's worth $500. This means I'm a brilliant businessman.
-bm
This is exactly what happened, and is still happening, with Bitcoin. Every single day, people pay $500 for something (1 BTC) that less than a year ago, everyone agreed was worth ~$100.
Many assets appreciate and depreciate with time. Automobiles depreciate. Fiat scrip depreciates. Art appreciates. Cryptocurrency appreciates. That's what makes these things sound investments.