Bubbles are incredibly difficult to pinpoint. Only in hindsight does it become clear.

Look no further than Bitcoin in 2013. Bubbled up to $1200 and popped down to under $200 thanks to Mt Gox. In the rear view mirror it appears obvious. At the time, it wasn't. Lots of the sentiment you hear now is exactly the same as what was being thrown around back then.

I'm not saying we're in bubble territory, but I'm not naive enough to dismiss the idea completely. What happens if China government regulation comes down hard? What if it comes out that Bitfinex is conducting massive fraud? These things could cause this to come tumbling down really quickly.

Being skeptical of current valuation after a 10X run-up in less than a year isn't necessarily a bad thing. Being blindly loyal is.