Why Does my cat miss my dog?

I got my cat in 2015, nine years after getting my dog. At first they didn't get along, but as time went by, they adapted to one another and became close, with the occasional barking or hissing, but never bitting. My cat loves going outside, my dog used to love sitting in the kitchen. The night before my dog died, my cat sat with my dog all night in the kitchen, then followed her in the living room and then garden. But now my dog is no longer with us, my cat doesn't play, seems tired and waits for my dog to come outside.

What is wrong with my cat? She's eating and drinking as normally, but just seems bored.