From the submitter:
"Uh. I mean. Yeah! How dare you forget! Buy me a new car?"
So the sender, who's listed as "Dad," calls the recipient "Dad" as well? My theory is that recipient adopted a child at a very early age - let's say he adopted a 2-year-old when he was 5 - and to help his son cope with the scant age difference between the two, the father just told the son that they were both each other's daddies because they had each conceived the other at the same moment in some quantum wormhole. The dad went to such great lengths to preserve this white lie that he listed his son as "Dad" in his contacts list. In this exchange, however, the dad finally tells his son the truth.