When I die, I won't enter eternity, as my concept of eternity has to come from a conscious being. When I am dead I am not conscious, therefore not only will I not be able to conceptualize eternity, but I won't be able to conceptualize anything. This includes pain, pleasure, emotion, etc. So then why do we fear death? Why then, do we grieve for people? I feel that to a certain degree it's instinctual, but I also think in many cases we rationalize our grief for what we call "lost potential". If someone dies when they are twenty, then we feel as if they missed out on their chance to experience the rest of their life. But why should the rest of their life have any more value than death? If someone lives a full life, then dies, why should we say they achieved anything more than the person who died when they are twenty if in the end they reach the same fate?
I do also think that a common reason for grief is that we won't be able to interact with the person again, or create new memories with that person again. Although, once again, we are grieving for this "lost potential" (albeit in a more self centrist {non-egotistical} way). And once again, it feels like we are assigning value to things that don't necessarily have any.