In the short term, price inflation can happen due to an increase in the velocity of money, which can happen based on crowd psychology.

But in the long term, in a free market society not at war (at least), prices go down in real terms not up. Because we get better, faster, cheaper, more industrialized, greater efficiency, etc.

Another exception that just came to mind is scarcity.  Some things get more and more scarce and those will go up in value.  Real estate, rare minerals, etc, go up in price due to supply constraints.

But the primary reason that prices go up in the long term is monetary inflation.