How does printing money cause inflation? I've had this doubt for a long time and I still can't find a proper answer to it. Most answers I get just partially explain the phenomenon of a general increase in price level. I want to understand WHY and HOW monetary issue causes inflation.
I'm always told that if a country A has a total amount of output of $100 and this amount is represented by 100 coins of gold, then each coin of gold stands for one unit of output. If one day I stumble upon a gold mine and produce 100 further coins of gold, then I'll have 200 coins, so each coin now represents 0.5 units of output. It's is completely obvious that now each monetary unit has a lower purchasing power, but how do suppliers notice this? How does the economy as a whole detect this increase in money? If the answer is an increase in the demand for goods, then why do suppliers raise their prices? Why don't they just keep prices on a the same level if they know inflation is going to destroy us all?
WHY and HOW monetary issue causes inflation.

When a central bank emits money (or decrease key interest rates), commercial banks will be able to refinance themselves at a lower cost. Which means that at the end, these commercial banks will also be more, say, flexible, and thus will stimulate demands (here and there) in the national economy by lending money (here and there). If supplies stay constant, markets will reach a new equilibrium via prices rising due to these stimulated demands $\implies$ inflation.