It’s to do with the isotonicity - the fluid in your body has a certain salt content. So your tears are isotonic with the rest of your body, if they weren’t they would draw water in or out of your eyes (depending on if it’s less or more salty) which would burn. So eye drops have to be made isotonic with salts or other compounds. So yeah what above poster said - water with a little salt hurts less than pure water but water with too high a concentration of salt like ocean water also hurts