What chaps your hide, boils your blood, or makes you hot under the collar?
I’ve read every one of the comments above, and each of them touches something real in me.
And having observed myself and pondered this question for half a century, I think it all comes back to injustice for me.
And for me injustice includes want, ignorance, any form of treatment of another that is less than respectful, any unthinking damage to the environment we all share, any action by any subset of humanity that is less than cooperative and tolerant of diversity towards everyone else.
And ignorance is perhaps the hardest, as it often appears as what seems like its opposite, righteousness. Science and logic are both clear, that we have no basis in reality or logic to be absolutely certain of anything. The harsh unrelenting certainty of “Truths” displayed by so many (in both scientific and religious domains) is to me the worst and saddest form of ignorance.
I am clear that at the basis of all of our knowledge and understanding are sets of heuristics (rules of thumb, useful approximations, things that worked in practice often enough to be useful) selected over the deep times of biological and cultural evolution. Absolute certainty about anything is almost always self serving delusion.
So I have learned to cut anyone who is basically cooperative and respectful a bit of slack in their beliefs.
One of the things I have the hardest time with is Hell Fire and Brimstone preachers terrorising young children. I have a hard time restraining myself from inflicting on the preachers the physical equivalent of the sorts of mental abuse they are inflicting on the children.