Greed is the worst problem of trading and unfortunately, 5 percent of traders that succeed are only successful because 95 percent of others have failed to meet the right decision. It is almost impossible to be among those 5 percent 
Greed is without a doubt a problem but if it is was not for greed then you will not be investing in any market so it has its uses, to me the biggest problems are simply ignorance and laziness, people want to obtain huge profits out of this market but they want to do it without studying how to trade at all and they refuse to do so simply because they are lazy and when you put all of those factors together then it is not difficult to understand why only 5% of the traders succeed on their goal to make money in the markets.