After the prank there was always a surprise treat, a prepared nice thing that showed how much they were loved and wanted. The pranks kept us emotionally able to accept setbacks and shocks - the after party was how we made sure they were healed within.
Those we prank, we thank, for being good sports and letting others laugh at them rather than with them. We thank them as publicly as they were pranked, we show that they are part of our loved "tribe" of humanity and big hearted enough to allow others their fun.
If we only prank those we love, then it becomes a sort of compliment, another form of "Valentines."
For a prank to have real humour it needs to come as part of a culture where pranks an accepted way to show love and acceptance, such as a nick-name or family chores. It has to show that the person is "one of us," rather than the opposite.