Since there is no central bank or any other centralized decision-maker to command the price of bitcoin or, more precisely, the exchange rate of bitcoin, it is you as well as other participants on the market who decide what the price of bitcoin should be. You can come on the market and try to sell your coins at whatever price you want, but you will soon realize that if you want to sell it quickly enough you should set the price as close as possible to that, which most of the market participants agreed with. The price is constantly changing and fluctuating because sometimes someone sells their coins cheaper than the actual price, or buys coins at a higher price. It all depends on how hard you want to sell or buy.