Why, do you want more reasons? Allergies, bulimia, stress aren't enough :-)

- Stress related muscle tension.
- Poor body posture.
- Poor vision.
- Loud noise.
- Sinus infection.
- Too much sun
- Extreme cold
- Pre-menstrual or menopause.
- Hunger.
- Too much alcohol.
- Certain types of food.
- Side effects of medication.
- Insomnia.
- De-hydration.

http://www.medicalnewstoday.com/articles/72024.php