A general one: Once a dwarf finds a lover, said dwarf will stay with said lover for the rest of its life. Even after the lover dies, the dwarf will not find another.
Something about having a tavern full of people from all over the world chatting, telling tales and poetry, and sometimes even forming an improvised singing troupe for everyone to enjoy, tends to cheer one up. As some in the forums have said, .42 is the first release where you can see your dwarves are truly alive.
As will traveling warriors who decide they really like the place, and ask for permission to stay. If it's granted to them, they will gladly fight to protect the fortress when danger comes.