Maybe just Occam's Razor -- any time I've seen Sam talk in public he just seems to be a neurotic, anxious individual that would have a hard time interacting with people in any normal context. In a world of infinite variables it's hard to say that his aversion was due to your race -- there's really not much to go on here.
Why would you think Occam's Razor would not determine that racism is the reason? It's just as simple of an explanation as anxiety.
I'm not sure some people understand how "normal" racism is.