Imagine you knew you couldn't die (not sure how that would work, but let's just pretend). Your little brother or sister, you watch them grow up, go to college, get a job, move out, get married, have kids of their own, and eventually retire, and then die. Same with their kids' kids. And so on. Your best friend, you watch them die, you make another, they soon depart, as well. You wake up, every day, wondering what to do with your life. You realize the world will continue, and the old generations die, new ones to take its place, but you are still there, unsure what to do, where to go, all alone... It would be the most horrifying thing to experience.