pain is the main, recurring physical reality. But pain affects emotions (you might walk around with a frown or grimace), the mind (you are constantly reminded of pain and can think of little else), and the spirit (pain does wear one down).
As Orwell notes, you just want the pain to cease so you can get a respite, a sleep, some comfort, some release, and any amount of freedom from pain.
And pain is, essentially, a personal thing; it is difficult or impossible to communicate its severity to others. You only know and really experience pain on your own terms, through your own senses.
In the end, pain potentially becomes the main and only enemy when individuals suffer from whatever injury, cancer, or condition. It is quite isolating and one ceases the usual person one is or was. Pain changes, limits, and often defeats people despite all our wonder drugs.