Because your pet did not agree to international bureaucracy. Yet here we are.
For most people moving to Spain, the dog or cat isn't an optional extra to be dealt with after the furniture. They're coming. Obviously.
They sleep on your bed, cost you a ridiculous amount of money and have absolutely no understanding of why you are suddenly discussing rabies certificates over breakfast.
The good news is that moving pets to Spain is perfectly manageable. The less-good news is that pet travel is one of those things where almost right = wrong. Microchips, vaccination dates, certificates and entry requirements need to happen in the correct order.
So here is the practical version.
Which pets do these Spanish entry rules apply to?
The standard EU pet-travel rules mainly cover dogs, cats and ferrets. If you're moving birds, rabbits, reptiles, rodents or exotic pets, different rules can apply and you need to check the Spanish requirements for that particular species.
And if you're relocating something considerably more adventurous than a Labrador, please do not assume this article covers it.
Moving a dog or cat to Spain from another EU country
For non-commercial travel between EU countries, dogs, cats and ferrets generally need identification by microchip, a valid rabies vaccination, and an EU Pet Passport.
Spain does not normally allow the entry of dogs, cats or ferrets younger than 15 weeks under the usual pet-travel rules.
Why 15 weeks? Because the minimum age for a first rabies vaccination is generally 12 weeks and at least 21 days must then pass before that first vaccination becomes valid for travel.
So no, unfortunately, "But she's tiny and adorable" is not an exemption recognised by border control.