bitcoin is not like gold, the reason why currency using gold as a guarantee because gold value is stable. And gold has limited amount.
Is bitcoin too ? you can answer that.

and that's why bitcoin has it's own way to increase or decrease the value.
a couple months ago, I remember bitcoin price at $700,- and now even more than $2000,-.

I think, it's purely about supply and demand, but the negative thing is, there is chance for someone to monopoly the price, someone or some people who have a lot bitcoin or resources.
Why I say that ? because until now, for myself, I don't even know what makes bitcoin has a value except that everybody want it.