It isn't easy to single out one specific cause of inflation but there are some sound explanations that do a good job of describing this phenomenon. Before we get into the details you need to understand that there is no clear cause and effect explanation. Inflation does occur, we see it happen in real life but all we can really do is observe this and make our best estimates as to why it is happening. Fortunately, there have been a lot of smart people studying this for a long time so we have a pretty good idea on why inflation happens and what the likely causes are.
The money supply is thought to play a major and important role on the effect of inflation. As the supply of money in the economy increases (i.e. the amount of money in circulation), the pressure on the economy to \"keep up\" and increase output also rises. This leads to price increases, because if the amount of money is increasing then its value is decreasing. It follows then, that if the value of the money is decreasing then you'll need to spend more in order to get the same amount of goods and services which is the definition of a price increase and hence inflation.
There are other models of inflation including: