Speculation, based on predictions and speed of adoption (main streaming). Many people within the btc community expect it to be as high as 5k by fall. But, even half that would be  interesting. We shall see nonetheless, for now it's nothing more than 'speculation'.   

Optimism is key imo. Playing the BULL market based on speculation drives the BTC market up in price. As long as good news and more main stream adoption continues the price will continue to rise.