If you try to buy a present for an
adult, if it's something they want, they'll already have
it, which means you're left with buying them something they
don't want. If you give them money, and they give you a present
(even if it's something you don't want), your gift is
deemed worse because money takes less thought. If you both give
each other money, but you don't give the same amount, it's
also a problem. If you agree to give each other money and you agree
on a specific amount to avoid problems, you might as well just keep
your money and pretend you swapped it.
