From what I have experienced, almost all traders tend to do this mistake in trading, which have adequate many of them loose alot of money.
Not all lost in trading is as a result of greed, but most cases I have come across are because of greed, and wanting to gain more.
Doe the primary aim of trading is to make earns, and gains but just as you said, as a good trader one should have the ability to listen to your inner voice.