User blog comment:B214/Question and Answers 6/@comment-118.172.205.177-20160711144501/@comment-24.113.252.213-20160712031247

Yes, Issei did accidentally activate the Juggernaut Drive with his negative emotions, but I don't think that means "it's activated by negative emotions."

I'll explain:

The JD was likely created by the collective negative emotions of the past possessors, since their lives were essencially taken over by the Two Heavenly Dragon's power. Since they possessed the power of dragons, they attracted attention, good and bad, and ended up suffering as a result. This resulted in the possessors having intense resentment for the Heavenly Dragon's power, and the Sacred Gears (Boosted Gear and Divine Dividing) created the JD.

What I think happened to Issei is that his intense negative emotions over Asia being killed (though it turned out she was alive), and his desire for power to kill Shalba Beelzebub, somehow "linked" with the past possessors negative emotions, awakening them, and forcefully activated he JD. What I think makes Issei's JD and incomplete one (as Vali said in vol. 6) is that it was the Boosted Gears "reaction" to Issei's feelings, not a voluntary activation.

Another way to look at it is this: Issei's accidental activation of the JD was like trying to open a gate, but you can't open it completely because the chain is still holding the doors together. You need a "key" to unlock the chain, and Issei has the "key." He himself has to intentionally remove the chains so that the gate can open completely.