Greed is a common problem. If you give a person a mountain of gold, he'll want another one and on and on. He won't be satisfied and if he loses it, he will be in regret.
It is hard to know what price and rate of profit to sell at e.g at 1X, 2X, 10X etc. This varies for each person and depends on their goals in live and what they want to achieve. Without determining this for oneself, one can't have a strategy for when to buy and sell, so one ends up going more on gut instincts and emotions.
Many also fail because they use TA and assume that past performance/trends prove future performance/trends. Such people tend to see what they want in the historical data, when actually the comparison is more complicated because the causes of the historical trends/performance are different to what is happening now. This is also bad for the market because it becomes a self-fulfilling prophecy.