By being greedy, you certainly think that the price will keep on growing that's why you decide not to sell. However, if you don't act upon seeing your trades are not doing good anymore, then you could experience more losing if you don't decide to sell at a stop loss. Using stop loss is not actually accepting that you are already losing your trades, but its the best option that could help you out from the big losses that you would encounter later on.
Still a great deal of traders do not really understand why a stop loss can be a very useful tool and refuse to use it, and unfortunately there is no way for them to learn, other than the hard way, about the usefulness of a stop loss.