The proverbial expression has nothing to do with Advent. Here's Wiki:
The Dog in the Manger is a fable attributed to Aesop, concerning a dog who one afternoon lay down to sleep in the manger. On being awoken, he ferociously kept the cattle in the farm from eating the hay on which he chose to sleep, even though he was unable to eat it himself, leading an ox to mutter the moral of the fable:
People often begrudge others what they cannot enjoy themselves.