I guess I am an eternal (or should I say "infernal") smiler on the mat, and am constantly making friends with people who I might have thought had an attitude. I love aikido, and love training with people who love aikido. It seems sometimes the people who appear to be grumps on the mat are really just being hard on themselves (and hence their looks of consternation). I certainly don't want to contribute further to their feeling angry or upset with themselves.
Now, that said, occassionally I meet people I just don't click with, no matter how hard I try (or don't try). That is okay too. And if it has something to do with the way I train, or my skill level, then it really isn't my problem to deal with, as I am who I am, and am here to constantly try to improve. Isn't that why we all practice?
An aside on the caste system comment; when people travel to seminars, there is often an area of the mat where the really gung-ho yudansha gather to thrash each other around. And often you hear comments on how the yudansha won't work with the kyu grades. I do see this happen, but I think it doesn't really have so much to do with them not wanting to work with the kyu grades, as much as it does that they haven't seen their aikido buddies from other dojos for awhile and really want to train with them. Personally I try to weave in and out of this group, train with them, and train with everyone else on the mat.
All the best,