All market price would be based on the law od supply and demand. Well for Bitcoin, it is a decentralized currency so no one is able to control it hence dictate the price or the market activity. Like any other market it is based on the law of supply and demand, but for Bitcoin, the supply is constant and limited, so mainly based on the demand. So it is the interest of people in it,if a lot of people are into it nad wants to buy it then the demand is high the price will be high, same goes if a lot of people are selling it and no one wants it, then the demand is low so is the price.