Oh hey, I've thought about this!
Luxu spent like a thousand years relying only on himself. It might have been something the MoM taught him, or it might have been something he came to believe along the way, but I think he thinks that you only need yourself to be strong. As a result, he believes that only someone who possesses individual strength is deserving of the keyblade.
Sora doesn't seem to take pride in his own strength, though. He's the total opposite. Even if he were weak and worthless on his own, he trusts in his friends to support him. After seeing the way the Foretellers utterly fell apart and didn't work together, it's likely that Xigbar doesn't see "friends" as a source of strength at all. The fact that Sora takes pride in something that Xigbar sees as a weakness is the thing that makes him react so strongly here.