Stop Loss is not an enemy but a friend and if one keeps this in mind one will be always raking money in the market. To put it in simple terms we can say that stop loss is a method of accepting a loss when is stock moves against you in order to prevent and kind of major loss.  In day trading one may not have had pre fed the same in system and may even keep it as a mental note rather than a pre-programmed amount because things are changing so fast.

Having a stop loss in position means that one is not subjected to unnecessary pain and losses because you didn't have a stop loss clear in your mind before you entered a position. As a strategy one can shift the position when stock hits the profit and can in turn convert a loss into profit. Thus if one is to be happy in day trading, than one has to have always an exit strategy!