How do you lose money in stock market ?

say you buy 100 shares worth 10$ each with your own money , no loan and margin , and the price drop to 5$ each , you lose half price on your shares , but is there anything else you lose ? if not anything else, why investor keen to sell their shares when the price drop ? , why don't they patiently wait ? , maybe the price could go up again , i mean there is nothing loss from waiting since he's invested with his own money , why don't they wait ? correct me if i'm wrong .