Toleration

Why are there some people you can barely tolerate, and there is no real reason why except they just grate on your nerves. I find this mildly unfortunate, but I'm not sure how to get around it. I find speaking to those people about as appealing as kissing a rabid raccoon. But I think what's worse, is that they never even know it. Let me ask this question...is it better to fake it and never hurt their feelings, or is it better to be honest?