"Runners high" is an actual, measurable physical effect. Running (and lengthy workouts, etc) releases endorphins. Everybody loves endorphins. The first couple times you're probably preoccupied with being tired, sore, out of breath, whatever, but after that it's definitely an elated buzz for a while after I'm done at the gym.

I think it took me about 3 weeks (going 2-3 times a week) to get in the habit where it wasn't so hard to get myself there and onto the treadmill, and over more time I did actually enjoy the cardio and I started spending longer on the machines. It also took me about three days of saying 'eh, next time' to completely lose it.