Common Courtesy

A decade and a half later, to effectively communicate on Twitter, I have had to consider how to communicate my thoughts effectively in 140 characters or less whilst maintaining coherence; this in turn led me to engage more deeply with the language of the emoticon and the visual attachment.
Why has common courtesy and good manners gone out the window? Is it because the world wide web is so impersonal that we don't think of those on the other end as real people? Or have we, as a people, just become more rude and indifferent and this behavior is now the norm?