Why do people criticize other people's spelling?

Why do people criticize other people's spelling or grammar especially if they cannot spell or punctuate a sentence properly? I'm aware that my grammar isn't always as superb as it could be yet some people always take minute out of their day to criticize me when I shorten a word or forget where apostrophes go. What angers me even more is that they are the people that think 'your' means you are and that there are no variations of 'there'. They constantly spell ridiculous amounts of words wrong, have horrifyingly improper grammar and they criticize me? So, my question is...why? Do they think they are some how superior because they're egotistical idiots? (Ahah, that's fun to say...)