It usually starts with an objectively attractive asset. As people raise their opinion of it, they increasingly want to own it. That makes capital flow to it, and the price rises. People take the rising price as a sign of the investment’s merit, so they buy still more. Others hear about it for the first time and join in, and the upward trend takes on the appearance of an unstoppable virtuous cycle. It’s mostly a popularity contest in which the asset in question is the winner.
