Become too greediness when investing can be dangerous for them.
When I hear trading. I hear greedy... so yeah greedy can become dangerous.. because a person should strive to be aggressive with his/her investment but not to become overly aggressive.. so if that is what you mean by the dangers of greediness, then I would agree with that perspective.
Some people became greedy its because they think they can ask for more profits from their investment and since there's a good tracks happen recently then they assume that there's a bull run will came that's why they usually fall down and lose their money for wanting for more profits. Scenario like this is normal when trading that's why its good for anyone to avoid taking those wrong decision and always be on point regarding on their investment since nothing will happen to them if they always feel greedy by the time they invest their money on bitcoin. Maybe best for them is to hodl but they need to learn first before doing this since being equipped with proper knowledge is really important before taking any decision either for deciding to hodl for long term or either for short period of time.