Headaches can develop for many reasons, from seasonal allergies and sinus pressure to respiratory infections, dehydration, and stress. Because these conditions can cause similar symptoms, figuring out what is behind your headache is not always easy. You might notice pressure around your forehead, pain behind your eyes, or a dull ache that gets worse throughout the day. The location and type of headache, along with your other symptoms, can offer clues about the underlying cause ...