Looking at this exchange, it kind of seems like there's been a misunderstanding, and you're both probably talking about different things.
Correct me if I'm wrong , but I think they were simply speaking from a mental health standpoint, i.e. how feeling an overwhelming sense of resentment and bitterness for extended periods of time can take a toll on your mental health. Not that we should pretend everything is fine in the world and do nothing, or that it isn't worth feeling anger against the injustices in the world. I kind of get what they mean, as I went through a similar low period some time ago before I realised that my extreme cynicism wasn't reaching anyone else but only hurting me. But yeah, it's understandable to feel this way when things are going to shit, or you're being hurt. I'd say it's human.
Personally what helps motivate me in times of hardship is looking back on what horrors my predecessors went through, and how thanks to their struggles and fights, I have the rights and comforts that I do have today. I try to push myself to do the same for the next generation of people, especially marginalised folks.