The tone, the mean-spiritness, discussing posters instead of issues. Even if you don't start that way, you become what you try to resist as a matter of survival. But some are quite mean.
However, some people just cannot deal with politics due to disagreement and probably avoid it for that reason.
“Experience should teach us to be most on our guard to protect liberty when the Government’s purposes are beneficent. Men born to freedom are naturally alert to repel invasion of their liberty by evil-minded rulers. The greatest dangers to liberty lurk in insidious encroachment by men of zeal, well-meaning but without understanding.” — James Madison