Placing a stop loss on each trade helps control risk, and also helps to avoid a common problem called "averaging down." Averaging down is when you make additional trades, adding to your position, as the price moves against you. Adding to a losing trade is a dangerous practice since the price can move against you for much longer than you expect.
Not only are you not exiting a trade that is losing you money, but you are making that position larger, which means the loss gets exponentially bigger the more the price moves against you and the more you add to the position.
Never add to a losing day trade. Take a trade with the proper position size and set a stop loss on the trade. If the price hits the stop loss the trade will be closed at a loss. There is no reason to risk more than that on a trade that is not doing what you expect. Small losses are much easier to recover from than large losses...