God allows suffering to bring about a greater good, His plan. He endows us also with free will, which sometimes means we make choices that cause suffering for ourselves or others. Free will doesn't mean all or even most suffering in a given life is because of our choices. Sometimes it is though.
Satan's playbook is all about denying these things, denying the cross, denying redemption. Satan is the one whispering that life isn't worth it, that it would be easier to end it, come down from the cross.
For a even better discussion of these things, I always recommend Life is Worth Living which is an old program hosted by Bishop Fulton Sheen. It is as relevant today as it was when he recorded it. Many of the episodes are on YouTube.
EDIT: many seem to be taking this as an anti-painkiller stance which it is not. Reducing pain until natural death is a great kindness.