It means that you like some certain dog: the one you see, or the dog of your friend, or the one you were talking about etc. It is perfectly correct, but it has another meaning than the first one. You should review your knowledge of the Article (the Indefinite and the Defenite ones), so you can understand their usage.
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cybercrystalA tiger is faster than a lion means all tigers?Yes. And all lions.
cybercrystalThe monkey is an intelligent animal means all monkeys?Yes.
cybercrystalWhen I want to talk about my love for dogs in general, I can say : I like dogs.Yes.
cybercrystalHow about this one : I like the dog.No. You can't talk about your love for dogs in general by saying this. This one is talking about one dog.