I’m 52 and learned to type on a Selectric typewriter at a time when we were all taught to put two spaces after each sentence. However, when computers allowed for proportional spacing between letters instead of fixed width, I managed to change my habits and use only one space because the rivers of white space flowing through the lines when you used two spaces looked horrible. I worked in book layout and publishing for twelve years and you get a good feel for why it’s less than favorable to a double-space after each line. It’s interesting that people will quibble that this is silly and not important, but I’m betting no one would argue that there’s no difference between a document or web page that is printed right up against the edges of a page (no or minimal margin) and one with proper margins. All page design choices are certainly up to the designer, but the idiosyncratic and often stubborn choices of any individual aren’t necessarily good for anyone or everyone. Design is about appearances and using two-spaces after each line tends to look poorer than single-spaced ones (and it feels like an enormous pause after each sentence when done in proportional spacing). Nonetheless, if you can’t break yourself of a trivial thing like incorrect spacing, then I guess you may have bigger issues than bad typography and should be encouraged to carry on as you have become accustomed.