A couple ideas:
Finnigan is on a diet and so he can't have his normal yummy treats and has only been getting veggies as treats. He still loves them, but some pups may be sad they have to eat their veggies.
Some pups could be sad they don't have dates for the YT Valentine's Day Ball
Both of my pups always get a sad face when they see me grabbing towels for bath time, clippers to trim their dairy-airs, or clean up a mess they just finished making.