There's cheap liquidity (ho ho ho said the FED and the ECB), there are institutions (pension funds) that are starting to buy in, and in comparison, the dot-com bubble (which was mainly a US led bubble, not global) went to $5.7tn to $1.7tn. Gold is worth $8tn globally. Why couldn't Bitcoin get to part of that level, e.g. $2tn?
It sure looks like a bubble, and it probably is, but it could literally be the largest bubble in our lives -- unlike one stock in the market, this is a global phenomenon + you only have a few currencies that matter.