Spoiler Alert
What would truly spoil a dog is if you broke his spirit, crushed his hopes, destroyed his enthusiasm. A ruined dog would no longer race to greet, beg to play, roll over to have his belly rubbed. But I have yet to see tender attention make a dog passive, bad-tempered, or even greedy. Their species is far better than we are at knowing when they have had enough (play, food, petting) and walking away to take a nap.