There ain't just one reason, but many...
There are many factors besides sentiments that decide where the price will go and why it changes, and it is mostly depending upon "manipulating bodies" who work behind the scenes with their huge amount of coins to make bucks on people like us...
The price fluctuates strongly in short time. We can not just simplely apply the economic rules supply and demand to it. IMO the main driving force should be from the speculative move. We should know that Bitcoin is a type of new currency and ppl are still sceptical about it! Its adoption rate is still not sufficient to make its price stable yet at the moment! We still have long way to go and make Bitcoin be able to compare with the current state of dollar!