In the US it's because the federal government gave a monopoly for printing bank notes to the Federal Reserve bank and they can print off as much cash as they want to (about $50 billion a week).
This causes a greater supply of money and it follows that the money is worth less.
The Dollar is based on a Global Valuation. If the above were even remotely close to being accurate there would be little cause to invest in the Dollar, and would render it worthless. Nations buy up the American Dollar as Investments, based on a tangible value. That value is based on it's worth in trade, backed by the net worth of the United States Assets be it Hard Gold or Credit worthiness. The Dollar is held as Worth a Specific Trade Amount adjusted daily against it's buying power and vast wealth of the United States, which happens to be far outside what a massive majority of the population can comprehend.
Simply "printing more" is a 3rd grade rendition of "where money comes from" and has no basis in anything close to reality. What value is there to you lending your neighbor $50.00 with an interest rate of 20% if all he has to do is simply Print the money to pay you back? He has to perform some measure of productive work to earn that original loan and interest in order to make that loan worth lending in the first place. There has to be a tangible value. His car, as collateral, is Worthless to you if all he has to do is Print his payment.
An example of the lack of recognition of "value" is the current understanding of the national debt. Soundbites like "Gee it's Huge" are correct if one balances it against their own material worth. If you consider the National Debt compared to an individuals Debt to Wealth ratio, it would be somewhat close to you earning 100K Per Year, owning your 500K Home Outright, and carrying a Massive $450.00 dollar Credit Card Debt. Run, Bankruptcy is a sure thing!
Inflation occurs because we are Not subjected to a finite wealth as some people would have you believe. There is not Pie that is divided up and if one person Gains, another must Lose. American Wealth is Infinite in potential, and the value of the dollar is based exclusively on the Daily Change of What's out there VS What can be purchased. If the Wealth slows and the products Must be sold, the cost will go down, and inflation reverses (DeFlation). If demand outpaces availability then costs are driven up and inflation occurs. The Feds will attempt to stifle "unfair" inflation by tightening up the availability of funds to loan out, in conditions where massive economic expansion ramps up the buying power outside a gradual sustainable climb. They do this by increasing the interest rate, increasing the Cost to borrow money, reducing it's desire and keeping it off the market.
I don't understand the Monopoly of printing money. Are you suggesting that Inflation wouldn't occur if Kinko's could enter the money printing business? Where is the Value in the dollar? Are you thinking that just because it's Printed it has an intrinsic Value? In fact, the Feds do NOT have a monopoly on printing money. Have you considered buying Canadian Dollars? Japanese Yen? Gold? Silver? Formica Futures? Land? You do know you have the freedom Not to use some bill with the face of a current King or Dictator exclusively, don't you? You are free to purchase and invest any way you like. The Common Currency of the US is Dollar Bills. You are free to stock up on Gold, and exchange Daily. One day you might get $300.00 Dollars for your ounce, the next you might get $310.00. You might trade your 1 Dollar for a Canadian Dollar, and the next day get $1.13 in exchange, you might very well get .38c, but what does the Fed Printing have to do with what the Canadian Dollar or Japanese Yen are worth? Those Also are valued based on their nations worth.
So, printing is the key eh? Care to take another stab at it?
Most people consider the Dollar itself as the thing with value, it's not. The Value is what the Dollar can Accomplish backed by it's Worth.