You notice your dog scratching energetically at his neck and you think to yourself, "Oh my goodness, Rover has fleas!" So you check him all over, looking for ticks as well, and find nothing: no flea dirt, no parasites. Puzzled, you give him a bath anyway and apply a flea and tick preventative.
That evening you notice Rover not only scratching, which you guess is from flea bites that leave itchy bumps like mosquito bites do, but you observe him rubbing his head on the floor and on the furniture. "Maybe he has an ear infection," you surmise, remembering what you have read about various symptoms and their causes. You check his ears: no redness, no greasy look, no smell.
You make a mental note to watch your dog carefully for the next few days.
The next unusual activity you notice is your dog lying down, licking and chewing on his front paws. Again, you think of some simple reason. Maybe he has a burr or grass awn stuck in his paw but he doesn't. So you give up guessing and take him to the veterinarian for an evaluation.