Most often the cause is viral (like the common cold) will last one to two weeks and resolve on its own. It is uncomfortable, causing a burning or sandy foreign body sensation.
Also occasionally bacteria can cause conjunctivitis (usually there is whitish or yellowish eye discharge) and require antibiotic treatment for more rapid resolution.
There is also allergic conjunctivitis which is due to allergy to perfume, dust, pollen, cosmetics etc and resolve upon the use of ocular antihistamine.
I suggest you check with an eye doctor if it persists to determine the cause.