I have always believed that love occurred between 2 people when the conditions are right. While you can't make a spark where one isn't, you can certainly work on those conditions.
While your dog isn't thrilled about reading a sweet card maybe consider dropping by the pet store and bringing a bone, a treat, or an annoying squeaky toy home for the loyal Valentine.
Love the gift. Flirt with the gift. Ask the gift about their day. And it’s very likely you will take the gift home and unwrap it, and better yet...you’ll wake up next to that precious gift in the morning.