[AnimalMuppet]

Prices go up for some reason. That's not a bubble.

People want to buy that asset because that's where the returns are (because the price is going up). Those people drive the price up further. That's still not a bubble.

People see that the returns are really good in that asset class, so they borrow money to buy into that asset class, so a ton more money moves in, limited only by banks' willingness to lend against that asset class. Now it's a bubble.

And the problem with bubbles is not that people lose money when they pop. It's that people lose borrowed money when they pop, and if it's a big bubble, that threatens the banks, which can threaten the whole economy (not just that asset class). And, because people invested borrowed money, as the bubble starts to pop, they panic sell, which drives the price down further, which leads more people to panic sell, so the whole thing comes apart very quickly.