OK, I know the question sounds a little bit extreme, but I'm writing a story in which there are two races of a same species, both rational and civilized, with cities and everything an actual society has. Both races are pictured as educated and not prone to violence (they're nice, but serious if needed people)
The thing is, both races hate each other, there are people among each race that would go to war if they can and support it, but there is no one who is OK with the other one, like... the most pacifist guy would still sit on the far side of the bus if they see a member of the other race in the front (that was just an explanatory scenario, they don't share cities.)
The thing is, the hate they have on each other must be well founded, it should be so that the reader doesn't think "Oh, there is NO way they can be like that", and it must be about race, not politics or religion. Also, it can't be a historical thing (Like, because of something that happened wayyy in the past) because they have the cultural level and the ethics to let go of the past if that was the case.
I was thinking of maybe something biological, but I don't think a virus or disease might work because I don't think it's possible for they to have evolved to the point where they share a planet on equal grounds (no race is noticeably poorer or richer in resources or education). Although, I'm open to suggestions on other areas aside from disease.
Finally, it can't be simply "nature" or because they are pretty much like humans, they can ignore their nature because they're rational.
EDIT: I'm going to give more details because I feel that some people who answered didn't get an important part of the question (probably my fault, I'm not from an English speaking country)
The idea here is that the reason not only isn't present in our world, but it's most likely that it is impossible that it applies to our world. Many people say in comments that it doesn't have an answer because I'm restricting the answer so it doesn't involve politics, history or religion, but I never said anything about geography, biology, chemistry or any science. Because its hard but very important in the story is why I'm pretty much open for you to do anything with the world, for example, does your reason need for them to live in separate continents? Sure, go for it. Thanks again for everyone who has answered, you have given me a lot of possible reasons I haven't considered.