Originally Posted by ThermionicScott

(You'll have to pardon the anthropomorphisms, but they make this easier to write...)

The way that I understand it is, the body is not attempting to make you "high" or "happy" when it releases endorphins or other analgesics. The goal is to strike a balance between anesthetizing pain and keeping the person/animal lucid and functional, so the body is trying to release just enough. So it's very possible that it is doing that, but you'll only really notice it on the occasions that you have pushed yourself especially hard, or given yourself a slight (or not) injury that required a little more. Occasionally, an uncontrollable smile will creep across my lips when I'm climbing a hill and I will be in really good spirits during and after the ride.

In the case of some of us, the effects of exercise are more notable in its absence. My girlfriend notices how content I am when I've been riding regularly, and how "grumpy" or unmotivated I might be if I haven't been able to in a while. Some folks are even able to cut back on (or eliminate) antidepressants because the exercise is restoring the mental balance they might not otherwise have. Whether it's due to endorphins or something else, it's a very real effect.