It is interesting that the name "Endorphin" contains the letters "orphin" like in "Morphine". This was intended when creating the name "Endorphin" since it is somewhat similar to "Morphine" in both chemical and metabolic respects. Endorphin can be released in when pain is experienced. So possibly we are having a pleasant experience when relaxing into pain during a pin in Aikido that is similar to the one when smoking opium. Still the problems caused by addiction to opium seem not to be present when doing Aikido. And certainly many other facts are important. So saying we just are addicted to Endorphin is far too simple, but an effect one should consider too.