I'll try: we have absolutely no idea to what degree any given living thing can feel pain, in the subjective sense. We know that mice, for example, react in a way that looks like pain to painful stimulus, but we don't know if there is anything that it feels like to be a mouse in pain. We care when humans feel pain, in a large part because we know, almost for certain (although not quite), that they are feeling pain... why should the same not extend to a creature that may be having a similar experience?