Because you can never really tell whats going on with any company behind the scenes. And when you dont have an exit plan it allows emotions to take over. So when your bullet proof stock is tanking because insiders are selling due to the company falling over, you're probably too busy finding more money to "average down". And then starts the slippery slope of not knowing when to get out and before you know it you've done you're doe and become another statistic.
If I dont have a preset stop loss, I dont trade/invest.
