[thaumasiotes]

> Doesn't the supply of stuff also matter?

It matters.

> How about people's expectations of future prices?

These also matter.

Now ask yourself how they matter. Any fall in the value of money must come from an increase in supply or a decrease in demand. If we expect future prices to be much higher than they are now, we will seek to hold less money (because its value in the future is lower) and this drop in the demand for money in the present will drive down the value of money in the present.