It's not normal when a normally happy, loving dog is suddenly aggressive for no apparent reason. Pain, fear, and illness can all cause sudden behavior changes and temperament changes like aggression. If your dog has suddenly woke up on the wrong side of the bed, you need to check him out completely.
- Locate the source of irritation. Try to find out if he's annoyed by loud noises (possible ear or head trouble), if eating makes him grumpy (mouth or teeth problem), or if he gets snappish when you get too close to him (he's in physical pain).
- If he seems to be in pain, start with a gentle physical exam. Look for swelling, cuts, torn paw pads, insect stings, tender spots; anything that is out of the ordinary.
- Check the inside of his mouth. I know it might be a daunting prospect if he's feeling snappish, but it could easily be something as simple as a piece of food caught in his teeth. If he's threatening to remove your questing fingers though, the best thing to do is bring him in to have a vet look at him.
- Take note of any and all other possible symptoms (vomiting, diarrhea, if he won't eat, if he's lethargic, etc.), and bring this in to the veterinarian's office with you.