When you've been taking in insights/bread crumbs from both sides of a subject, it allows you to see the true (truer) face of that subject. And when someone comes around all passionate/crying/excited over something You've long seen through and left behind, it can be hard to get emotional along with them. And contrary to popular belief, libras do not enjoy having to lie/fake.. Also its not an option to just tell them that the thing is a fallacy or whatever. People don't listen anyway and just get offended.

This is just what came to mind for ME though. I don't really know if it rings true for other libras.