We keep our eyes opened longer when reading, driving, or doing many focused activities. This reduces the blink rate, and leads to aqueous deficiency. If there is meibomian gland dysfunction (MGD), or blepharitis, with poor quality, or quantity, meibum, the oily layer of the tear film does not effectively spread over the eye during blinking, leading to pain or discomfort.
Conjunctivochalasis, characterized by the bunching up of conjunctival tissue, can cause extreme pain while reading. Some people with conjunctivochalasis refuse to read anything at all because they know they will experience extreme pain and discomfort if they read.
Too many tears, whether due to reflexive tearing during reading, or because the tear ducts are plugged or cauterized to retain moisture when there is aqueous deficiency, can cause pain and other discomfort, such as a feeling of acid, when reading.
Any foreign body sensation can make reading difficult, not only because it is distracting, but also because sometimes a foreign body sensation can cause the eye lid to flutter or stay shut, making reading all but impossible.