Other than the dream of being rich, and a general belief in the potential of bitcoin, what exactly is the reason for a rising price? Yes bitcoin is useful, yes it's novel, yes crypto currencies are likely to be a feature of the future economy. But really?
Precisely? More purchasing volume at market than sale volume at market. People either want their bitcoins right now to spend, and can't be bothered putting a bid at sale price and waiting, or speculators don't want to miss the train. The former is value. The latter snowballs.