A lot of my dwarves have had personality changes from seeing a certain kobold die (probably because his body is still just sitting outside where everybody can see it), and while a lot of the personality changes make sense (increases in sadism, becoming quick to anger, etc.) but some don't necessarily -- I saw one whose positive outlook was due to repeated horrified memories of death.
I'm not saying that's a bad thing, in fact, I almost prefer it this way -- people will have very different responses to the same event. However, we should try to do some research (or just get the info directly from Toady) on what experiences can change what traits and in what directions, if possible.
Can also confirm that getting caught in the rain and drinking water without a well are some of the greatest personality changers in the game, simply because they are so common to every dwarf and happen so often. Again, not necessarily a bad thing, as repeated experiences of something different than what you're used to will change your outlook on life. The issue is when these two behaviors collide, and you get multiple, strange, and powerful personality changes just from drinking stagnant water too many times.
Like other people have reported, little thoughts like these also tend to be the greatest (and most difficult to satisfy) contributors to unhappiness and distraction. In my current fort, an engraver who constantly relives her husband's death is considerably happier than a fisherdwarf who mopes about not having enough to do, getting caught in the rain, being away from family, and being away from friends.