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.
You can buy something because you believe it will be worth more in the future because it would be more useful. Like shares of a company that is small now but will become large in the future. Or with Bitcoin, it will be more adopted as a currency or store of value.
The problem with Bitcoin is that there's no good way to calculate how much it should be worth like you can with stocks, it's all just pure guess and the fact how much money can people actually pay for it. Some people say that Bitcoin will be worth $150,000 or $500,000 - but if they truly believe it, why won't they sell all their assets for it?