@drahardja Why are we talking? Do we want to change minds by discussing things with others, or do we want to shout at them? Granted, both are necessary at times...
But if we want people who disagree with us to listen to what we have to say rather than ignoring us or shouting right back, then we ought to show them the respect of being polite. That does not prevent us from speaking truths that they may find painful.