Ok scrolling down quite a bit here we go:
As a further testimony from the Western Church we may quote one of the many passages in which St. Augustine speaks of prayers for the dead: “The universal Church observes this law, handed down from the Fathers, that prayers should be offered for those who have died in the communion of the Body and Blood of Christ, when they are commemorated in their proper place at the Sacrifice” (Serm. clxxii, 2, P.L., XXXVIII, 936).
Note that St Augustine says “the universal Church” (that’s all of us) observes this law (law), handed down from the Fathers (so in place from earliest Christian times) that prayers should be offered for the dead. (and indeed in the Holy Mass during the Eucharistic prayers we offer prayers for the 'faithful departed/ all who have died".
Since as Catholics we are obligated to attend Mass on Sundays and holy days of obligation, we pray for the dead as an obligation as well.
Now there is more than one Eucharistic Prayer in the OF (legitimate ones along with ad libbed ones, don’t start me). . .and all of them mention prayers for the dead. All of them.
It’s as if God is saying, “Ok, I know you’re busy people. I know that you might not have time for prayer most days, except in praying you get through whatever you’re going through :D, so I’ll make sure that at Mass you have the chance to pray for everybody you should, and who, if you actually had the chance in this busy, frantic world with all your other obligations, you would think of and pray for”.
Now I have a question – are we obligated to pray for the living?