The numbers in your brokerage account invite you to pretend that you are a seller at times when you are not. It's a nice fiction when prices are going up and a dark story when prices are going down but it's just as illusory either way. We are in violent agreement that price action is psychology, the point of the quote is to fight the loss aversion bias that was calibrated for hunting animals in the savannah not trading stocks. It's a point of fact that if you're a net buyer, gaining the option to buy at a low price outweighs losing the option to sell at a high price, and if you let the latter psych you into not taking advantage of the former you are doing yourself a very common, very human disservice.
But the people complaining the most about falling prices are the sellers, of which there are necessarily many. (If there weren't, the price wouldn't be falling!)
It's not a flaw for people to think from the seller's perspective. For you to be a net buyer, someone else has to be a net seller.
Not to mention the unhappiest of all: the people who borrowed money against the value of the assets they owned.
