basically it all comes down to supply and demand. what affects those though is a whole mess of things from the news to other markets economys and all kinds of stuff. basically anything that is remotely connected to btc will affect the price .
I agree to this one, of all the answers. I also think that price varies directly from the law of supply and demand. It is the people that affect the price of Bitcoin. Every day, more people join and get involved into it, while the supply is very limited. That causes the changes that is happening to the value of Bitcoin over time.