The same as any other thing. It is the basic concept of supply and demand.

When the demand for Bitcoin is low and the supply high, the price would likely face a fall; correction if you wish. I would urge you to buy Bitcoin then as you can save quite a bit of money.

When the demand for Bitcoin is high and the supply is relatively low, the price would skyrocket. We would experience a price hype. I would urge you to sell Bitcoin then as you will make the most profit.

Another thing, if the demand remain the same, the supply will eventually fall as the Bitcoin block reward halves. Another great characteristic making bitcoin a deflationary currency.