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
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.