Prayer for the Dead
When a friend or relative tells you that they are unwell or unhappy, the natural reaction is to tell them that you'll be thinking of them. This helps them to understand that they are not alone.
For the Christian, we will very often say that we will pray for someone who is need because we would like them to know that our care for them goes on after we have gone our separate ways. In other words, we would like them to know just how much we care for them because we will continue to keep them in mind.
If someone is unwell then when we pray for them, we are often saying to God that we would like them to get better, but if they cannot, then we are saying that we would like them to know they are under God's protection and do not need to feel free afraid of anything they might be facing.
When someone dies, they have passed beyond our sight and beyond our immediate care. A Catholic perspective takes the view that although gone from us, no-one is ever lost to God. Just as when someone is alive, we pray and care for them so when someone dies our prayer and love continues. There is no argument against prayer for the dead that cannot equally be used against prayer for the living and - as Jesus said "all men are in fact to God, alive."
And so we pray for those whom we have loved and lost, we pray that they may know the light, refreshment and peace of Christ. We pray that their virtues may live on in the way we approach our lives. We pray that they may know the power of our love and prayer.