Acharide Wrote: Demand and supply, there's only 21 million bitcoin and more and more companies and customers want to have them so the price goes up
This is pretty much as accurate an explanation as can be provided. I'd like to add that BTC in general is a pretty stable currency not only because of what acharide already mentioned (it's widespread use which leads to more and more people accepting and using it) but also because it is not traceable and is not governed by any central entity. This means that a. governments and law enforcement cannot track the transactions b. they cannot regulate the currency whether that would be adding more, less, or imposing taxes.