Although my knees prevent me the pleasure of running may I suggest you have become a drug addict.
Three main pleasure drugs are released during exercise to give you pleasure and euphoria to varying degrees. Dopamine, serotonin and endorphins. Luckily, none of these is illegal and all are produced naturally by the body.
Enjoy your running. It can only do you good.
Can't say I've ever enjoyed running. It was something to endure at school, I tried jogging for a very very short while whilst at Uni, both experiences painful. I did cope with the treadmill at the gym when I used to go. But it's all rather masochistic.
No, running's not for me. With short legs, long body and slightly overweight I look like an elephant on the rampage.
Now, give me a bicycle and I'll pedal for miles every day. Normally pedal about 10-15 miles a day and go to the gym 3 times a week. Kinder on my hip and knee joints as well.