"For every trade, there is a buyer and a seller. The buyer thinks the stock is
going up, the seller thinks the stock is going down. One of them is wrong."

A generalized simplification that is often not true.

The seller not necessarily sells because he thinks it will go down. Often it is
because the expected profits have been made or a better opportunity now lies
elsewhere. I often do that with my short to mid term trades.

While on the buyer side I do so for a long term hold or for short term if the sp
has recently retraced on one of my favourite stocks. In both cases the stocks
must have good fundamentals and psychology has little to do with it. Just numbers
and timing.

I suspect the points posted have to do with general, i.e. inexperienced,
investors rather than regulars who have gone through the grind.