Stock market is a 0 ending game, if someone wins that means that someone loses also. If you think that the price is reasonable to buy yourself in and you expect the price to go up, someone at the same moment thinks that the price will go down and it's high time to sell. After it, one of them will win and the other has missed the opportunity to win, so it's a loss for him.
Stock market is not a 0 ending game. It is positive sum game. The reason for that is most companies make profit, this money will be paid to investors. The trading cost, tax will reduce the return to investors.