When trying to analyze an animal's body language, each signal must be taken in context. Other body language signals should be analyzed as well to form as complete as possible view of the animal's intent or emotional state. You can generally characterize signs in 3 categories: go, stop and yield. 

Dogs can put their ears forward for many reasons. Generally, this means that they are alert or focused on something. They could be focused on a squirrel that they want to chase or a person to whom they may be aggressive. It is best to interpret this as a yield signal which means that you should try to analyze the situation and look at the other body language signals  to determine the best next steps.

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This dog has his ears so far forward that there are wrinkles between the eyes. He is showing an intensity toward the subject on which he is focused--in this case, the veterinarian. When taken in the context of his other body language cues such as dilated pupils and a tense body, it is clear that this dog doesn’t want to the veterinarian to interact with him. He is likely to show aggression.

The dog in the picture to the above has his ears forward, however they are not pushed forward enough to cause the skin to wrinkle up between his eyes so he is not as intense as the dog above. His ears are slightly to the side and his face is relaxed. His pupils are of a normal size for the light in the room. He is alert, but not aroused. He is looking out the window.