From Getting Started with Dwarf Fortress: Learn to Play the Most Complex Video Game Ever Made, by Peter Tyson, published last year by O’Reilly Media, Inc.
When a “strange mood” strikes, a dwarf will stop whatever they are doing and rush to a workshop, their heads filled with an idea for a legendary artifact that they would like to construct. If the dwarf is able to successfully complete the artifact, they will often become legendary in the associated skill, gain a happy thought, and your fortress will have a valuable artifact to use and admire.
It is highly likely that at some point along the route from strange mood to artifact, something will go horribly wrong and you will end up with a dwarf camped out in a workshop doing nothing but flashing “!” impatiently. This is a problem: while in a strange mood, a dwarf won’t eat, drink or sleep. He will eventually go insane and may even start randomly attacking nearby dwarves.
Dwarves can form grudges against other dwarves, and having to talk to a dwarf they have a grudge against causes an unhappy thought. Caution: “miasma” is a buildup of stench and gas from rotting or spoiled plant or animal material. Dwarves encountering miasma will experience an unhappy thought.
“Tantrum spirals” usually start when one dwarf snaps and starts a fight with another dwarf, either killing that dwarf or being killed in self-defense. The death of a popular dwarf (or worse, two popular dwarves) will result in a flood of unhappy thoughts through the heads of many fortress dwarves.
Try and reduce these common causes of unhappy thoughts: hunger and thirst; encountering miasma; not enough chairs in the dining room; accidentally murdering a friend while in a strange mood; encountering ghosts; seeing another dwarf die; sustaining injuries; having no clothes.
Dwarves who are forced to drink water get unhappy thoughts and work slower than happy, alcohol-fueled dwarves. Brewing ten stacks of alcohol will keep your dwarves happy for a month or two.
It is inevitable that you will lose a few dwarves as you walk the path to greatness. What you do with the dead dwarves you accrue is quite important to the health and well-being of your fortress. There are three reasons for this. Firstly, rotting dwarf bodies lying around your fortress tend to make living dwarves somewhat distressed. Secondly, rotting bodies cause miasma. Finally, dead dwarves who aren’t appropriately respected by their surviving brethren will come back as ghosts and haunt your fortress. We can avoid these problems by building coffins at a Mason’s Workshop in which to place our dead dwarves, and installing the coffins somewhere convenient. Once this is done, you will probably also want to toggle “Allow Pets” to “(N)” so that your coffins aren’t filled with dead cats.