Many traders are smart enough to know that they will win in the long run by taking only the best trades, but become impatient due to the lack of anything happening. They forget that the long run can be "long".
Taking only the best trades can be frustrating. They come along rarely, and even when they do there is no guarantee of winning with them. Long gaps occur in between. We can become angry and irritable, and our emotional balance can be severely tested.
Many traders, especially beginning traders, seem to think that they have to trade all the time if they want to make money. Very often the opposite is true. A good trader is a patient trader, because he knows that "the long run" is longer than we think.