Dragons generally hoard treasure out of pure magpie instinct, but Spike had a better reason. They were all means to an end, the end being acquiring the one treasure he ever really wanted, throughout the years of being held as a servant and plaything in Ponyville, where his growth and natural instincts were suppressed by magic. When he finally managed to break the bewitched bonds, he had but one goal left.
And Rarity? She was nothing if not pragmatic, and having access to the vastest treasury in the land more than made up for certain draconic embarrassments at high-society events.
A little dark and evil, lol.. been meaning to paint this for a long time now.
Rarity is one of the ponies i like less, but nobody can say i discriminate