Those like myself who learned to type on a typewriter learned double spacing because a typewriter uses what is called monospaced fonts, meaning all letters/spaces use the same amount of space. Therefore it made sense to use more space between sentences. Computers use proportioal spacing, so for instance an "i" uses less space then an "s". The "space" bar uses more then (I think) anything else. So while it is now widely accepted that single spacing is correct, I for one missed the memo and continue to double space between sentences.
http://grammar.quickanddirtytips.com/spaces-period-end-of-sentence.aspx