Simple: The stability.
Stability makes Bitcoin not look like a Ponzi-scheme. Stability makes Bitcoin look like a viable currency. Stability reduces the chance of monetary loss.
As a result, more amounts of Bitcoins are being bought from many more different people, and spent.
So yeah, stability makes the price rise, destroying stability
![Cheesy](https://bitcointalk.org/Smileys/default/cheesy.gif)
Nope. Buying does not necessarily increase the prices.
If person A is selling 1 BTC for $100
And person B buys 1 BTC for $100
The price
will not changeThe price will only change if both
I ) Sellers sell for higher prices
II ) Buyers are willing to buy for higher prices
And more importantly, if a greater amount of users come and they each buy less, the price will also not rise. If a few people buy a ton, the price will probably rise.