why is it that cats swish their tails when they are unhappy but dogs wag their tails when they are happy?

Question: why is it that cats swish their tails when they are unhappy but dogs wag their tails when they are happy?
- Keywords:
  cat,
  dog,
  emotion,
  tail
Asked by anon-22441 to Charlotte, Jo, Kevin, Louise, Valeria on 19 Jun 2012. This question was also asked by rebeccam.
- Kevin Mahon answered on 19 Jun 2012:
  
  For dogs, it’s a remnant of their ancestors which ran in packs – the tail wag was a simple form of communication, showing that they were friendly and meant no harm.
  
  In cats, it is a way to show agitation, and has as a means of telling others to keep a way, likely a result of their ancestors more solitary lifestyle.

Question: why is it that cats swish their tails when they are unhappy but dogs wag their tails when they are happy?