Emotions in trading could cause you a big stress but on the other hand it's hard to be fully cold, we are not machines. The key is find some kind of balance between cold mind and the heart because sometimes you just have the feeling if something is wrong or right, we have all experienced that.
One emotion that you never, ever should give a chance is panic, this can really cost you a lot and it will totaly blind your common sense.
Well this is the emotion that wins for most traders, especially newbies and weak hands, when all they need is to calm down and think thoroughly what to do. Yeah, seeing your investments loosing profit is sad, frustrating and etc but if negativity wins you over then you will lose more than what you have already. Trading is fun, exciting but the market is very playful and it will play you, will urge you to have mixed emotions at the same time.