Girls respond better to people they like.
(I.e. they are more likely to think you’re funny if they like you.)
Girls are more likely to have positive feelings (i.e. like) towards people who like them.
Boys are more likely to buy expensive things for people they like.

Ergo:
Boy buys Girl shiny. Girl thinks this means boy likes her. So she likes Boy more. Boy is more appealing. Thus Boy is more funny.
QED.

Either that, or you’re not very funny and need to buy diamonds to become appealing.