If you don't, I'll try to explain. I will talk about goods only but everything is equally applicable to services as well. Let's assume that the amount of money is constant. If the amount of goods increases that would necessarily lead to price deflation, i.e. you will be able to buy more goods with the same amount of money (or a unit of money will buy more goods). If the amount of goods decreases, that would necessarily lead to price inflation, i.e. you will be able to buy less goods with the same amount of money (or a unit of money will buy less goods). Both of these cases will cause imbalances in the economy simply because the changes in the value of money (i.e. how much a unit of money can buy) in real life don't propagate instantaneously through the economy
That would lead, for example, to some people getting unwarranted advantages over other people since they could either buy goods cheaper than they should or refrain from buying goods dearer than they should
Why are fluctuating prices a bad thing? If there is an over-supply of something, prices going down would dis-incentivize production, as they should.
The important issue is not whether prices change, but what causes them to change. If the cause is market-based, it should be healthy and/or self-correcting. The problem with central planning of money is that prices are changed system-wide not for market reasons but due to what is ultimately market manipulation to benefit the elites.
For example, an ounce of gold used to buy a nice tailored suit when it was worth $22. Today that gold will still buy that suit, but try buying it with $22. Over the decades, prices always go up, because the elites generally make monetary policy too loose, to allow themselves to receive unearned wealth and power from issuing public and bank debt.
An even more damning example is central-planning-driven deflation. All financial crises happen because (A) too much paper wealth of a certain kind has been issued and propped up (indirectly) by state power, and (B) markets have lost faith in the value of that paper. To alleviate the economic pain after the financial bust, what central banks can do is to generate massive inflation. (This in effect would acknowledge past mistakes and start anew.) But what they do is to generate just enough inflation to keep the political system stable, but never enough to avert major pain. This was what led to the Great Depression and Great Recession, plus countless examples outside the US.
The reason is that massive inflation would expose the long-term working of their system to public opinion and ultimately cause savers to go to non-state-issued money, like gold, in which case the elites will lost most of their future power. The adopted policy contradicts the elites' own moral code of 'alleviating pain at all cost,' as propagated by modern mainstream economists. Apparently, such a code is only good for justifying policies aimed at prolonging asset bubbles by any means, including deception, during the period of asset inflation.