I find when I don't exercise, I turn into a right bitch :)
And if I've had a bad day, it really helps if I can even get to the gym for 20 minutes and do something...I feel soooo much better and forget all my issues for a little while.
You are all correct in sharing your 'elation' with cardio workouts. Increasing heart rate and hence blood/oxygen circulation also releases "endorphins' (good hormones) that actually help you "feel good". The longer you engage in cardio work, the more 'feel good' hormones are circulating in your body. So when you're down and out, and just want to feel better, lace up your athletic shoes, get up, and MOVE :)