Elizabeth Brydges was 14 years old when Hieronimo Custodis painted this portrait in 1589. This alone would account for the mischievous look on her face, but as it happens, she was already serving as a maid of honour to Queen Elizabeth I. Though she would by most reckonings then have been considered a grown woman, think about it: any young creature thrust into that much excitement and importance is going to enjoy it to the possible fullest, while dodging any number of complications. That fabulous dress isn't just a status marker, it's armor.
Luckily Custodis (Flemish, worked in England, died c. 1593) must have had some empathy for this youngster so carefully managing herself, for he included a scrappy dog jumping up on those fine skirts. More often than not, the inclusion of a dog in portraits of women serves as a reference to fidelity and loyalty, which a maid of honour would need to show her sovereign. Yet it doesn't hurt if that pet's animal spirits mirror those its owner is still fresh enough to feel, even if she's already wise enough to keep them in check.