The last thing Shaggy needed was a ghost.
He was already haunted by- that thing he wasn't thinking about.
He couldn't even drown his woes in food anymore. Not after-
Nope. He still wasn't thinking it.
But every sound he heard reminded him of- No! Not thinking! Forget!
He heard a howl in the distance. This reminded him.
He heard the ding of a microwave. This reminded him.
He heard Fred whispering to Daphne. This reminded him.
He heard the scratching of paws on the floorboards. This reminded-
Paws? There were no paws in his house!
Scooby was haunting him. Through his tears at the reminder that Scooby was- gone…
Shaggy cried in morose joy that he could behold Scooby again, despite his lack of a physical body.
A ghost dog would do. Velma disagreed. Shaggy didn't care.
He could see Scooby again, could talk to him. That was enough.