Not enough research, trading for the sake of it. Refusing to take losses. Small profits, big losses are a few reasons that pop to mind.
That, IMO, is a very important factor. It is surprising how I will take a loss of 10 points and be reluctant to close at a 10 point profit. I want it to go higher and, when it starts to run back instead, I take any kind of profit, even at breakeven so as not to take a loss. What about the times that it starts to reverse on the opening, though? Taking the profit does not arise and the 10 point stop is triggered in the end. This means a lot of mediocre trades, plus trades that are stopped before the good one comes along, if it comes along.
It is a very difficult problem to solve and if the trader is not careful it becomes, quite often, a death of a thousand cuts.