I think over time most players become desensitised to all the violence, and stop really caring about their dwarfs. I know when i first started playing i did my best to try and prevent my dwarfs from dying, making sure they all had high-quality bedrooms and so on. But as my fortress grew and more and more deadly events occurred, i stopped caring about individuals dwarfs. Instead of thinking of the typical individual dwarf as a lovable, unique character with their own little quirky traits; i started to think of most of my dwarfs as just another useless idiot, and when they get killed by some random carven creature i just think, "one less nitwit to deal with", and spare it no more thought.
In addition to this, something else i have noticed is that as my fortress has become increasingly safe and secure, i am starting to long for chaos and madness to liven things up. While in the early days of a fortress it is difficult and therefore entertaining to just try and survive(with this closeness your fortress is to destruction also making the life of any one dwarf more valuable), in later years you reach a point where food and booze and dwarfs are abundant, and your military can easily wipe out conventional threat(A squad of legendary melee Dwarfs armoured and armed with high quality steel can easily take out sieges full of goblins without losing a single man ). Alas, after a while success becomes boring, and you begin to wish for something interesting, something FUN, to wipe out your fortress in a dramatic matter, and having this happens becomes a lot easier if you are reckless and careless.