"This is the generation of the great LEVIATHAN, or rather, to speak more reverently of that mortal god, to which we own under the immortal God, our peace and defense." -Thomas Hobbes: Leviathan
For I am sure that neither death nor life, nor angels nor rulers, nor things present nor things to come, nor powers, nor height nor depth, nor anything else in all creation, will be able to separate us from the love of God in Christ Jesus our Lord.
The reasoning I think is rather simple. Death itself does not pose a barrier to the love of God nor separates us from it; it is therefore legitimate to ask for an application of God’s love and grace for the dead. Being dead does not place a person beyond the pale of God’s love.
The divine Word did not overcome death so that we may be struck dumb with no words of prayer or hope for the dead. It is precisely to overcome death whereby the hope of the Gospel was revealed. What separates us from the love of God is not death but sin, and even then, sin is overcome by divine forgiveness. Therefore, the dead are not beyond the pale or hope of the Gospel, but the dead are more especially the hope of the Gospel, that death is not the final word for the Word has triumphed over it. Prayers for the dead are therefore not only permissible, but it is intrinsic to the Gospel which proclaims the victory of the Word over death, which Word cannot and should not be silenced, but be enacted in prayers for the victims of death.