User blog comment:B214/Question and Answers 6/@comment-118.172.205.177-20160711144501/@comment-24.113.252.213-20160712034957

Dude. Saying the Biblical God intentionally created the JD is like saying he intentionally created the Balance Breaker, and the Longinus (plural).

All these things are "bugs" in the Sacred Gear System that God created. He made the Sacred Gears respond to their posessor's/wielder's wills and emotions. He never intended for humans to advance the Sacred Gears to achieve BB, or for some of them to become Longinus. It makes sense that the JD was created in a similar fashion.

And I said the JD was "likely" created by the past possessors negative emotions. It was a theory I made that made the most sense.

I still maintain what I said about the "chain and key" in my earlier post. Like you said: "Issei's still capable of showing sadness and despair." Issei didn't completely give into the JD and was still somewhat himself. He didn't intentionally activate it, so there's still a "chain on the door."