WHy do we feel good after exercising?

Question: WHy do we feel good after exercising?
- Keywords:
  exercise,
  exercising,
  feel good,
  positive
Asked by hazellwh to Helen, Jenni, Mark, Martin, Stu on 18 Jun 2011. This question was also asked by venusjwh.
- Mark Burnley answered on 18 Jun 2011:
  
  This is a great question. The most common explanation is that exercise releases endorphins that lead to what has been called a “runners high”. In reality, there are so many metabolic and hormonal adjustments that occur after exercise that many things could make you feel good. Because it makes you feel good regular aerobic exercise is recommended in the treatment of depression. But treating depression isn’t something I know anything about!

Question: WHy do we feel good after exercising?