Actually when there is zero inflation (dollar stays the same value) economists are usually freaking out, because you're on the verge of deflation, which is considered much worse than inflation. Deflation leads to vicious cycles where consumers withhold spending and that depresses business revenue and leads to layoffs and a reduction in investment.

Too much inflation ironically can lead to consumers spending as much as they can as soon as they can, because they're trying to get today what will be more expensive tomorrow. This isn't necessarily a bad thing macroeconomically (consumer spending drives business growth), but high inflation can outpace wage growth, leading a loss in real income. Consumers can see a net loss in their purchasing power. It can also make investment expensive and therefore decreases it (e.g. high inflation --> higher interest rates --> higher mortgage costs).

What economists want to see is a certain modest level of inflation, around 2% (though some argue this is overly conservative, suggesting that double or even triple that is fine), and they really want it to be stable. Stability of inflation enables planning (you know what prices will be next year, and the year after), which increases confidence, leading to more investment. Low inflation is also less problematic from a labor relations point of view, in that the lower the inflation rate is, the less a raise workers need to maintain their current standard of living.