Getting to the point, bubbles happen when you start buying something just because you think someone will pay more for it, instead of looking at how useful is it for you or for a community of people. Buying something is actually telling the market that they are wrong, and what you are buying is worth more than what the consensus means. This is being contrarian, anything else is speculating and creating bubbles.
The ease of saying "bitcoin is in a bubble" comes from the fact that it doesn't really have a value or a cost and that is the reason. Sure miners have a cost but they create that cost, if every miner in the world stops right now and only you mine it with your laptop that will still be valid (albeit very inefficient) and you will get it all and make a killer profit. Sure the more the merrier for the efficiency of the project but that cost is still insanely lower than what it is right now and never really reflected on the price before, we had times when cost was higher than the price and we have times when price is higher than the cost.
The real cost of diamonds is known to be very little, or tulips are like a dollar at most, whereas when it comes down to spending 1000X more on them, people did it and still doing it and that is the real reason for calling it a bubble.