I've tip-toed around this for awhile, but it's becoming more and more pertinent that I put my foot down on this sooner rather than later.
Joshua is really very intuitive with his emotions. He's like a child that can't cover up what he's feeling at that moment, so he's always easy to read. He fumbles on instinctive human needs, like the desire for friendship and love. What I'm trying to get at is that Joshua is something like an advanced four year old emotionally. The real Joshua got tuned out while still being an acting consciousness in the background, and effectively a new Joshua was born. A Joshua that went to live amongst demons, got spoiled to beat all, and only really has a zombie maid that cares about him. ...You try not to come out maladjusted.
However, it's not just nurture that makes him up-- It's nature. Joshua as he is now acts almost as the anti-thesis to who he was before. He's bold, up-front, and brash where he was hesitant, caring, and reserved before. For good cause though, it's shown in canon that being able to connect with his past and his true character means who he is now loses control and ultimately becomes his own undoing. Compassion, love, and other sappy feelings that Joshua was full of before were slowly buried and locked away to the point where it takes a shot to the head before he can tap back into them.
Even with someone like Fiore, who he's especially close with, there's a sense of hollow affection. Joshua grew up depending upon her and doesn't pretend or play false that he needs her desperately. However, he still purposely makes a comment that he knows will startle, or even hurt, her at one point with a self-deprecating smile. He does move to comfort her, because he is aware of what effect his words have but he doesn't reassure her that he loves her or anything like that at all.* He just shows the same reliance and thus concern for her that comes from that. Even when when he senses she's gone, he's like a child abandoned, but nothing is shown beyond a faint sense of loss.
The cut and dry: Joshua can not do a lot of things that are based in and around love, like friendship and the like, because he doesn't have ability to. He can't be upset, beyond being really pissed off so waterworks or being hurt don't happen. He can't be happy beyond being amused at others expense or being able to occasionally socute at kids. He is hollow. He's unconsciously keeping away from what will unravel what he's become now, but he hasn't grown enough to understand why he's reaching for companionship in camp which is where a lot of his frustration comes from as he tries to shun that urge.
* The fact this small bit of honesty between them is done while they aren't looking at each other is intensely significant and interesting.
God, I think that accomplished nothing. Questions or comments go here. Another essay may be quickly forthcoming, who knows. I kind of fail at essaying Joshua, orz.