who is the person who says that today it should be worth 40k and tomorrow 42k?
In summary: Buyers and sellers.
Bitcoin doesn't differ much from every other asset, product, commodity. It has a demand. It has a supply. It has an equilibrium price. It has a equilibrium quantity. Whenever the demand for a product increases, so does the price and the quantity that the buyers and the sellers agree upon.
For example, if people suddenly want to get rid of their bitcoin, because, say, they feel anxious about what's happening in the world, they will be willing to sell it lower than the price they were willing to sell it previously. That's a decrease in demand, which will drop the price.
One cool thing that's different in bitcoin is the production. Normally, if the demand of a product increases, so does its
PE and
QE, and if the supply of a product increases, the Q
E does too, but the P
E decreases. This means that if there's a concurrent increase in both the demand & supply, P
E is counterbalanced.
The supply curve of an
inelastic product, such as diesel, looks like this:
(Displayed linearly for the sake of simplicity) The supply curve of bitcoin is perfectly inelastic*, parallel to the Price Axis:
That's because the supply of bitcoin cannot be altered due to a change in the price. There will be 6.25 BTC brought into circulation every 10 minutes, then it'll be halved to 3.125 BTC etc. Therefore, when there's an increase or decrease in demand, there's nothing else that can equilibrize the price. That's one of the reasons it's highly volatile.
*Note: Again, for the sake of simplicity, we can assume the supply is the coins in circulation and not the exchange's liquidity.