Providing a better living situation for a distressed animal out of compassion is good... I can't see the fault in that. Examining the arising of attachment to pets, and finding the letting go there, as practice itself is good - I don't see the fault in that. But I also don't see the fault in not creating stress for oneself either, by not engaging in a recognized stressful situation!
And if you do feel the inclination to help troubled dogs, there are ways to do that without owning one.
Hmm so carefully examining one's own mind and acting only on the faultless motivation(s), that's the way to go, no? But what that means specifically (dog or no dog) ... listen to your "heart".
But not acknowledging craving for a dog, if there is craving - then that's a fault, and so not a good idea to act out of *that*.
"When one thing is practiced & pursued, ignorance is abandoned, clear knowing arises, the conceit 'I am' is abandoned, latent tendencies are uprooted, fetters are abandoned. Which one thing? Mindfulness immersed in the body." -AN 1.230