They don't "have to" always grow, but stock prices are effectively predictions on future growth. When the prediction changes you see that reflected immediately

They don't "have to" always grow, but stock prices are effectively predictions on future growth. When the prediction changes you see that reflected immediately in the stock price. Of course predictions change all the time and so do stock prices. If your prediction turns out to be more accurate than other predictions you win (assuming you invested some money based on your prediction).