My dog loves rubber balls - slightly smaller than tennis balls and made of solid rubber.
She rolls her head against them, licks them, cuddles them, and enjoys rolling them under large pieces of furniture only to try digging them back out. She even brings them with her to bed at night.
She's not very good at bringing them back, however.
I thought dogs loved to fetch.
1st World problems
She's just not that into you.
Some dogs get very weird about certain toys, and balls seem to be a favorite fixation item for some. I've known two dogs like this. They stare at the balls when they're out of reach, they hide them and keep them in their crates or personal space, and they seem neurotic in general about them. I don't know if there's a confused parental pattern slopping over into possession issues
The only thing I have been able to do is just remove any object that becomes a obsessive fixation. And that means not putting them up where the dog can see them. Out of sight, out of mind.
I've also seen dogs to this with stuffed animals and the occasional chew toy, but balls have elicited the strongest response.