Many of our habits are designed to keep us safe. When sitting in a car, I always buckle my seat belt. When crossing the street, I look left and right, then cross. In the same way, when taking a trade, I always put a stop loss in. It comes naturally, without effort.
While successful trading will ultimately be determined by profits, you will not be profitable on every trade taken. Therefore, to make money, your edge must give you a higher probability in profitable trades as compared to the losses suffered in losing trades. For this reason, your losing trades must be restrict your losses. It is for this reason that a losing trade may actually be considered successful if the trade was executed properly.