Why does pink eye keep coming back?

Problem redux: Either your eye gets reinfected or your allergy returns.

Blepharitis/Demodex : Pink eye in general means that you have a severe virus that causes significant itchiness. Most people however with a crusting, eye redness, lid swelling have an unstable tear film from blepharitis/blepharoconjunctivitis. See Eyedoc2020.blogspot.com "lid hygiene" on how to use warm compresses & diluted tea tree oil on closed lashes to help.

Likely re-infecction: Pink eye, also called conjunctivitis, can be caused by a virus, a bacteria, or an allergy. Each one presents slightly differently, so it is important to get seen and diagnosed properly to get the proper treatment. A few serious problems can also cause a red eye. If you have pain or decreased vision, be seen immediately. Blepharitis is infection of the eye lid and lashes, not the conjunctiva.