I'm (gratefully) not in extreme pain. It kind of depends on how you define pain though. If certain things about life cause you to suffer eg. Money worries, wage slaving and- you can't realistically change those things then surely, it's life itself you have a problem with.
Like- if I told the average person that just having to conform to life- working, maintaining a living environment, maintaining my own health makes me miserable- they'd likely find that a pretty flimsy excuse for being 'in pain'. That's just life- right? That's what everyone has to do. So- I'd say it's more the aspects of life that are unavoidable that I'm keen to escape from. Like you OP, currently I'm more at the level of being deeply fed up rather than being in constant pain.
I have been in genuine pain in the past though and, I fear experiencing more of it in the future. So- that's a other reason really. To try and avoid future pain.
But, it's a dumb saying I think. Like- life would be ok if only you won the lottery. If only you didn't have an untreatable illness. If only you were born the gender you feel. I feel like that saying is (as always) based on this idea that life is fundamentally brilliant for everyone underneath it all. You just need to fix a couple of problems and, you'll love it! It doesn't take into account that for many, their problems are almost intrinsic to their lives. Some problems are incredibly hard to fix or even mitigate.