Elves are unreliable. From a dwarves perspective, considering their society is based around a system where one death, one poor harvest, one strange thought can lead an entire fortress into a cascade of spiralling death, reliability is prized.
Elves ask for things. They negotiate. They even demand things. Sometimes they're your best friend, sometimes they're at war with you. But they also have things we want, so it puts us at a disadvantage. If we want their giant eagles, we have to be nice to them and trade with them. If we want to harvest the timber that is our's by right of occupation, we have to piss them off. Their strange views on wood make changes to dwarven economy that is simply unacceptable sometimes. Their caravans take just as much time and cause just as many problems for a fortress as any seige would, except they actually expect something in return as well as limiting what you can do in the meantime. Therefore, they have our hate. Sometimes they get killed for it. But they made us think about it. Do we want to kill them or not?
Goblins are reliable. They turn up, they fight. Or more accurately, they bring iron with the only negotiation possible being to kill them for it. They might "request" you to set up a fiendishly complex layout of traps, train your military or batten down the hatches, but they don't force you to. It's never a demand if you want their lovely, lovely goblinite. It's always on offer, they never stop bringing it just because you didn't decide to kill the last lot of them. They never tell you that they'll stop trading goblinite because you didn't slaughter enough things last season. And so therefore, they are reliable.
Humans don't get any ire, because they don't make any demands. Trade or don't trade, war or no war. They're not too sanctimonious about it. Similar to elves but without the hippy crap, but without the war animals either. There's not much they have that we can't get ourselves, or have a decent substitute for, but we can at least do what we please as we decide on whether they'll die or not.
Kobolds? Poor little buggers.........