Out of curiosity I just thought where do bitcoins came from??
They're mined, currently at a rate of 25 BTC every 10 minutes. In other words, "printed out of thin air" 
...
Just imagine you own 10 bitcoins and it's valued $400 per bitcoin now... if the total supply change, that value will go down, because more bitcoins will be
entering the supply.
As a holder of 10BTC, you have no say in the system. You can only vote with your feet -- by trying to sell your 10 BTC.Just imagine you own 10 bitcoins and it's valued $400 per bitcoin now... if the total supply change, that value will go down, because more bitcoins will be
entering the supply.
Miners mine BTC, and sell it. Their interests are not necessarily aligned with the holders of BTC. For instance, if they could double their block reward, and by doing so devalue BTC by 45%, it would be in their [pragmatic] self-interest to do so (BTC will be worth 45% less, but they'd make twice as much of it).