Working in business being nice and agressive without showing that much emotions to the other side will guarantee you success in most negotiations, not to mention building a more efective cooperation to achieve a common goal... most of the times being emotionless is the key, but when emotion is required "nice" is a much more constructive approach.
Now, if we are talking about real non-faking life... I'm a lawyer, and I think I'm nice... I'm a God actually, so I'm nice... I just tend to be upfront and honest about stuff and people most of the times dont like for other people to tell them the truth in the face...