I don't think, just too many people want to profit from the bitcoin, led to excessive bitcoi prices, but the bitcoi in the future could become international currency, so I think the bitcoi is not a bubble,
I think bitcoin is not bubble. Bitcoin is decentralised currency which means no one can control it. And Because of this its price is ups and down in time to time
When its price fall down many people consider it is a bubble. According to them like bubble bitcoin expand ita empire in past but now it suddenly ban or burst like bubble. Bitcoin uncontrolled production and distribution is the main reason behinds its volatility. In the year 2018 its price constantly falls and it is ban in many country too because of this above reasons many people thing it is bubble. But i think its price will be increase in near future.