My best guess would be fighting or just, well, stress due to cramped pastures. But I don't know!
Anyhow, I figure I'll kick off some more discussion, having just (for now) survived a siege in which around (if I'm counting right) 45 of my dwarfs (some children and babies) died, with a few pets, and there were 83 survivors. The besieging army was actually made up of one goblin and a bunch of dwarfs, so it looks like some innnteresting things may have happened in Worldgen.
Anyhow, a few observations:
Despite losing around 1/3 of my fort, I have not YET had any tantrums or suchlike, and it's been about not quite a season in-game. It could still be coming, of course, I'll cross my fingers anyhow that it doesn't get too bad.
I did have a LOT of canceled jobs at first, as different dwarfs were horrified, or suffering emotional shock. That mostly seems to have passed, although I still get a bit here and there.
Some specific emotions and what seems to cause them:
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"Nothing" (or rather "didn't feel anything") at seeing stray animals (or wild animals) die. Also found at least one dwarf that felt nothing about watching someone's pet alpaca die. Haven't noticed yet about pets from owners. Also got a "didn't feel anything about being in conflict" from a dwarf that "has trouble mastering his fear in a dangerous situation" oddly, of course, I don't know the exact details of the combat. (No wounds, so it must not have lasted long.) Also one of my dwarfs didn't feel anything after experiencing trauma.
"Horrified" at seeing dwarfs die. Interestingly, this included the invading dwarfs. And the lone goblin as well.
"Grieved" at the death of "somebody." As in, it didn't say who, or what connection they were. I'm assuming it was a relative or someone they were close to, since there are dwarfs with a whole boatload (as in almost a whole screen) of "horrified" reactions at seeing people die, but only about like two "grieved" notices.
"Shocked" after being attacked, or at the unexpected death of "Somebody." Again, it doesn't specify. And it does include "unexpected" in this description. Still not sure at the differences in what makes a dwarf grieve at a death, or be shocked at an unexpected death. (None of my deaths so far have been from, say, old age or anything, so they should all be "unexpected.")
"Afraid" after experiencing trauma. Not sure what that is specifically - if it means running away from enemies, being traumatized by deaths, or what. (This appears on dwarfs with no medical history, so it apparently doesn't mean physical trauma.)
"Empathy" after being yelled at or cried on by an unhappy citizen (for the mayor).
"Disgusted" after retching on miasma, or being forced to endure the decay of a child.
"Indignant" after being forced to endure the decay of a mother or sibling or pet.
"Shaken" after being forced to endure the decay of a father. (As before, not sure of the difference.)
"Satisfied" at work, or after crying on/yelling at someone in charge. Also after receiving water or food.
"Blissful" after becoming a parent. Also after sleeping in a good, very good or great bedroom or dining in a good, great, or fantastic dining room. (I don't have all that many dining rooms, and only one public one although I did add more tables/chairs to it recently, so I'm also not sure if the whole good/great/fantastic is a matter of set values, or the perception of the dwarf in question.)
"Content" after a bath or having a fine drink, or after sleeping in a good bedroom or dining in a good or a fantastic dining room. Not sure if content vs. blissful is a difference in the bedroom quality despite some under both emotions having the same "label," (ie "good") or if it's a difference in the dwarfs.
"Fondness" talking with a friend, or making a friend.
"Embarrassed" conducting a meeting in an awful setting, or dining in an awful dining room, or at a lack of dining tables, or sleeping on a rough cave floor.
"Gratitude" at being re-elected.
"Pleasure" near fine pieces of furniture. (Sometimes "his own," sometimes not.)
"Interested" near fine pieces of furniture. Again, I'm not sure if the difference is in quality (despite the same description), or in the dwarfs.
"Sympathy" after giving someone water or food, or bringing them to rest in bed.
"Annoyed" after sleeping on a rough cave floor, or at a lack of chairs, or eating without a proper dining room. Also after receiving a minor injury.
"Adoration" after becoming a parent. This happened in the same set of thoughts as someone that also felt "Blissful" about the same thing.
"Angry" after being forced to endure the decay of a spouse.
"Irritated" when drowsy.
"Frustrated" considering the scarcity of cages and chains (from the sheriff/captain of the guard)
"Restless" after resting and recuperating in bed.
"Guilty" after being able to rest and recuperate.
"Panicked" after being attacked.
"Hopeless" after suffering a major injury.
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Interestingly, while some of the really hard-hit dwarfs (emotionally) seem to be okay thus far (and in fact most of my dwarfs are fine), a few of them are getting the message "Over the long term, <name> has been under a great deal of stress" before their thoughts. Not sure what can happen if this doesn't improve, I guess maybe we'll see if I'm unlucky.
And on a side note, so far it seems that negative strong emotions are red (anger, horror, fear), mid-range negative emotions are yellow (panic, fear, shock, grief, disgust, hopelessness), slightly milder negative emotions are brown (annoyance, irritation, indignation, embarrassment, frustration, restlessness, guilt), and strong good emotions are green (bliss, satisfaction), mid-range good emotions are blue (interest) and lesser ones are aqua (sympathy, pleasure, fondness, empathy).
I could of course be totally wrong on those ratings or groupings, but it seems to be my best guess so far.
As another side note, I was trying to see if there was any rhyme or reason to dwarfs feeling "pleasure" or "interest" near fine furniture, or having no emotions about them mentioned. I noticed that thus far I haven't seen both "pleasure" and "interest" show up in the same dwarf's thoughts about furniture, so I thought it might be a matter of their personalities... but I found a few dwarfs that hold craftsmanship in the highest esteem and value their works, and they felt either interest or pleasure. Urgh. I will add though that so far, if a dwarf doesn't have an opinion on craftsmanship/etc., I haven't really noticed them noticing the furniture, so to speak. I could just be missing it though because my eyes are starting to cross, looking at all these emotions.
I saved at that point for now, because my brain is now melting. >_< Hope this isn't too annoying since it's a wall of text without probably too much useful, but it might at least start to pinpoint some trends and what can cause what emotions...? And also pinpoint some things that need a lot more Science! such as why different dwarfs have different emotions tied to the same type of trigger.