So we want to be in the safe area between high inflation and deflation, and below but close to 2% is exactly the right level.
Note that since 1998, when this number was chosen, most of the major central banks around the world have converged towards the same number: it has been adopted by the US Federal Reserve, the Bank of England and most recently by the Bank of Japan. In short, 2% inflation is some sort of a lubricant for the economy.