We say "politics is the mindkiller" but ti seems an seperate question why certain political topics are more 'mindkillery' than others.
Recent example that brought this to mind is the conflict going on in Gaza, its unusual in that my friends and acquaintances who are normally fairly moderate and willing to see both sides on political topics are splitting very heavily onto opposite sides, and refusing to see the others point of view.
Political issues are mindkillers because people tend to incorporate them into their personal identites; however, people often incorporate certain issues more strongly than others. A more general slogan might read "incorporating things into your personal sense of identity is the mindkiller".
If it's worth saying, but not worth its own post, even in Discussion, it goes here.