Somone else may have already offered these two possible answers, but here’s my best shot.
plssibility number 1: they have been taught incorrectly about just what Catholics believe, either maliciously or out of ignorance, and see us as idol worshippers that sell forgiveness in advance for the right donation among other things.
possibility number 2: They resent the difficulty of keeping the tenets of the faith, particularly precepts and/or obligations they find difficult to accept or understand and the authority of the pope. They don’t wish to have to change their lifestyle to be Christian, and find it arrogant that the Church would expect such. Some examples of such difficult precepts might include the prohibition on contraception, the absolute pro life stance, the lack of ordination of women (all of which are interpreted by the world as keeping women in servitude), the Church’s refusal to bend on homosexuality and other subjects that the world at large sees as hateful or discriminatory, and perhaps most importantly, the sacrament of Confession and the Real Presence.
I think the anger comes largely from the fact that the natural law is written on the hearts of men, and so, deep down inside we all really do know better. But admitting it means giving up all those worldly and selfish pleasures we become so attached to and accepting God’s will rather than insisting on our own.
I myself was once angry about the seeming many hardships my family has had to struggle with that other people don’t seem to have to. Now I can see that He has used these things to draw us to Him in a very special way and strengthen us.