Why do Christians grieve when someone dies?

[tonychanyt]

Same reason why non-Christians do. It is part of the mourning process. It is psychologically healthy, a cathartic release. Even Jesus wept when Lazarus died. Psalm 116:

15 Precious in the sight of the LORD is the death of his saints.

On the other hand, 1Th 4:

13 But we do not want you to be uninformed, brothers, about those who are asleep, that you may not grieve as others do who have no hope. 14 For since we believe that Jesus died and rose again, even so, through Jesus, God will bring with him those who have fallen asleep.

Christians know that physical death is not the end. We have hope in eternal life. Spiritually speaking, I can even rejoice in my death and my Christian friends' deaths.

In practice, both reactions can be exhibited.