For starters, it depends on the classic supply-and-demand. The more the people are willing to buy Bitcoins, the more the Bitcoin value will increase .
See this chart to better know.
As you see chart if more people are selling it its prices will dip.
This is how its value fluctuate wildly with time,The unpredictability of Bitcoin’s value makes it an extremely risky investment.
Before you invest in bitcoin, As a general advice DONT invest more then you AFFORD to LOSE