Greed is definitely the number one thing. They can invest sums that we can lose, but some of us invest amounts that are too big out of greed that they can't even afford to risk it. Greed is what continues to confuse other people, and eventually they still wonder why they have fallen for it. Imagine holding your money and then waiting until the price goes up, and all of a sudden the price begins to go up and you start buying lots of coins because you think the price is going to go up forever and you're going to become a millionaire overnight, haha, that's greed, you're taking those who've been holding a fool for a long time.
Greed without a doubt is the number one problem in this market, but it is inevitable, we are all looking to get more money out of of our investment, if that was not true then we will not be here and be invested in this market, what we must do is to control that greed and the only way to do that consistently is to have a very defined strategy about what to do in every single circumstance.
That way you're not going to let your emotions take control of you just at the moment in which you need to be more focused about doing the right move, if you are able to create a successful and logical strategy then most likely you will become one of the few good investors that earn money in this market.