Loss. And if you're up for doing 2 headcanons, torture. :3
Dan* doesn't really have much to lose, so loss doesn't affect him. That's what he thinks at least. Now that he's lost Dan, he feels this mixture of anguish and guilt and self hate. Not to mention he's putting himself in the position of everyone in his past lives that he left behind because of this experience, which makes him feel worse.
If Dan* is the one doing the torture, he is almost unnaturally calm, but positively ruthless. His favorite method of physical torture is electrocution, and since he has a way with words, he can deal a bit of emotional torture as well.
If he's the one being tortured, you won't get much out of him, whether it's information of screams. Dan* has a very high pain tolerance, almost to the point where he welcomes it.