The beauty of giving gifts

of course, this is a simplified equation, but you can get the idea:

if A buys x for A-self, A's pleasure is m
if B buys y for B-self, B's pleasure is n
so, the overall pleasure generated is (m+n)

if A buys y for B, and B buys x for A,
A's pleasure is m (from x) plus the pleasure of getting a gift to B
B's pleasure is n (from y) plus the pleasure of getting a gift to A
so, the overall pleasure generated is (m+n) + the additional pleasure of getting gifts to each other

AND, the monetary cost is just the same!!!