There's plenty of good quotes about this... but as of recent #LegendOfKorra has plenty of good ones... especially some of which... "Why do GOOD people suffer?"

The other one that I can't find directly quoting was about finding meaning in their suffering... basically to the effect of:

Episode 9: Beyond the Wilds from Legend of Korra Season 4 (Balance)

I guess I had to go through with dealing with that kind of suffering so that I could understand what it is for others who do... and by learning it makes them a more sympathetic/empathetic kind of person to be able to truly emotionally understand what others are going through.

The other that is notable is:
(Talking about Ang's Suffering)

Katara: But he never let it destroy his spirit. He chose to find meaning in his suffering and eventually ... found peace.
Korra: And ... what am I going to find if ... I get through this?
Katara: I don't know. But won't it be interesting to find out?