We have all done it - gone off our plan or failed to honor a stop that we ourselves set. The question is why? It's not like we don't all know that is what we have to do.
You are not sure enough of your system or you are undercapitalised and each loss is a pain. Now I don't think there is a single reason but many reasons depending on the context and the person.
Can people stop without understanding why they do it? Everyone knows smoking is bad for them too, but many still do it.
Either one of two reasons IMHO 1.) You're not preparing yourself to take a possible loss BEFORE you make your trade. I.e. you say to yourself that you are willing to lose X amount of money in order to make Y amount. If you don't prepare yourself, when your trade moves against you, you think gosh I can't afford to lose this! And so you hope and hold until it really goes against you, then depression takes over and you throw in the towel. 2.) Your ego won't let you admit you were wrong. However, if this were the case your ego wouldn't let you post it here - instead you would suffer in silence.