I agree to your point as there are factors that really change the mentality of some traders, both greed and fear that comes out when they are in the process and while waiting for the market to move, it's no an easy task following your set limits or following the strategy that supposedly you are planning to execute. Temptation and such fear inside you can change the direction of your target and may lead you losing your position and the amount that you set for your investment.
I think the reason is that it is a very difficult job that requires a lot from the trader, perseverance, discipline, and many other rules that cannot be broken. I think that in trading you can achieve profit, but often it is a small profit, because they are not ready to trade big money, or are carried away by futures, where I think it is very difficult to achieve positive results. Another problem is that everyone wants to get a quick result, and in trading this is unlikely.
That last part of your statement affects the outcome most of the time, most of those people who enter this industry thinks that it's aquick way to make money, instead of takingtheir time researching and analysing they tend to do it like betting on thier fate, thinking that if they manage to identify the next market movement they will get the amount that they expect to earn.
But in reality, those who think that way ends up losing a lot, fluctuation and all those market movements easily affects their thinking and since that they didn't established good trust to their investment they end up fearing and being shaken by any downfall and ends up following the trend.