Why are my eyes so tired?

Many causes. Many things can make your eyes feel tired. Naturally excessive near work causes the eyes to feel tired and this would include reading and computer work. The type of lighting used for these activities is important also. Dry eyes, allergies, frequent use of preserved drops, incorrect prescriptions or uncorrected refractive errors are other causes. Other eye diseases also cause this.
Most likely dry eyes. This is likely due to keratoconjunctivitus sicca or dry eye syndrome. You should try artificial tear lubricants three times a day. If no better you should see an ophthalmologist.

Related Questions

Why do my eyes get so tired after reading for only 15 minutes?

Refraction. Regardless of your age, if your eyes get tired after reading for 15 minutes, you likely require a refractive correction with glasses because the muscle inside your eye that changes the shape of the lens to permit you to read (accomodation) can only hold out so long before tiring. If you are younger than 40 you likely need this correction for all distances. If over 40 you may only require readers. Read more...