The realization that to some pet, you're merely a fallible god, but one who can often offer little but death in an alien place, filled with the scent of sick, dying, and/or terrified other animals. Why am I here? Did I do something wrong? And your face is the last one they see. Run through that a thousand times per year and, unless you have monastic levels of detachment, you might end up feeling as if you were little more than the keen whistling edge of a scythe which never quite dried.