You want your dog to be healthy, but it's not like you're the one eating the dog food so you'd think you're more likely to make a logical choice when choosing their food. That's not necessarily the case, as ads can target you very well even if you're making a decision for somebody else.
The ad pictured here (see more) depicts two skinny dogs engaging in human-like intercourse while a fat dog watches. The tag line reads, "LIFE'S HARD when you're a FAT DOG." This ad is designed to be funny, but it's also designed for people to think about how humans judge fat people and play on their desires to lose weight. The ad isn't selling a better sex life for your dog—a dog that is likely spayed or neutered—but playing upon human concepts of sex and beauty. Sure, a fat dog is likely an unhealthy dog but where in this ad do you learn why the dog food is healthy? You don't, because, again, the ad isn't targeting logic—it's targeting your emotions. You don't ask whether or not the dog food is healthy because the ad is asking you if you care whether or not your dog is healthy. These types of ads make no real claims. They simply identify the problem and you connect the dots. You assume there's a connection when there may not be one at all.
So what do you do about it? You do your research. When you view an ad, it helps to ask why. Why am I reacting the way I'm reacting? Why does this product solve a particular problem? If the product interests you, the answer should too. Look for product reviews (while being aware of fakes) and other information that can help you determine if what you want to buy can actually do what you think it can do. Don't buy blindly—do your research first.