Probably because Amity is all about peace and Dauntless is a lot about fighting. One would think that Amity would get along with everyone, but they be like: Peace and love...or else. :'D (They still annoy me the least; I don't wish to be offensive, I simply feel it is more accurate to say than "I like them the most". See how you can be both kind and honest? See?? Oh, nevermind haha).
Also, why are, for example, Candor only against Amity for their dishonesty? If Candor is made by only those who put honesty before everything else, doesn't that mean no one else does, hence everyone else is dishonest in no more or less amount than Amity? And the other way round: if everyone else value another virtue more than honesty, doesn't it mean they neglect it on that virtue's account just as much as Amity?