A bubble is not about how high is the price of something (tulip bulbs, diamonds, dot-com stocks...), it is when the price is not justified. It is a bit for everyone to decide if the price of an asset is justified really, there is not a magic formula that will tell you if it is. In stocks or investments you can make a "discounted cash-flow" apply a rate of return and make some hypothesis about economy, growth and "moat" and at least you will get a number from it. On other assets, the price will be fixed by supply and demand (and that is the case for bitcoin) since they do not produce a regular cash flow.
When it comes to decide if there is a bubble, you have to think about how much is worth to you. How much would you pay to get, e.g. a 4% in dividend instead of spending the money today in a car or a TV? How much would you pay for a diamond or even a soccer ticket? (my personal for those two is quite low)? Or, How much would you pay for 1 in 21 million bitcoins?
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.
I agree with you, bubbles can happen even if the price is not so high. The thing with bubbles is that most people thing they can get out before the bubble burst. It is like with a pyramid scheme. Most people are aware that the prices are higher than the fundamentals make sense but still go for a trade. It is all about finding a buyer that is willing to pay more than we paid, and then the bubble burst all of a sudden. One problem for comparing prices to our own preferences is that it is not universal, people will be giving different prices for a diamond or a soccer ticket. Some people want to get the cheapest ticket and are willing to sit in the back rows, while others want to sit every game in the first row and are willing to pay 1000+ per season.