Since Dexter passed away this winter, life at Amy and Kevin's has been different. We all know what it is like to have four paws missing in our house. Going from one dog to two is not easy. Dexter was a neat dog.
Whether he like it or not, Dexter's best friend was Sparky. Sparky could be a pain in the butt, but none the less they were best buds.

Just like people grieve in their own ways, so do dogs.

I guess the other explanation is that Sparky is only the alter-ego of Superdog.