Because, in general (and specific), when dealing with humans in a forum where emotions and feelings are paramount (which are most contexts) and a forum where we will need to interact with these people again (which is most places) it is far better to be nice (and taking into account the emotional state of the participants) than it is to be right (or at least to think that we're right).
This is so important it forms one of the five main human personality components, that is Agreeableness.
