I literally never thought about it. I never had a teacher in college or grad school who wanted me to single space after the period, so without any counter-influence to my early training on typewriters, the two spaces has stuck. (And this is despite having learned speed typing on a teletypewriter... although come to think of it, periods are also a matter of three keystrokes on those, as well.)

The printed page has always been different than typing, though, since the letters can be differently spaced. What really has changed though is the difference between paragraphs. No indentation? Just a double space down? My typing teacher would be rolling in her grave. (And yet I do it automatically.) Just as automatically as the doublespace -- which I'm typing, even if it doesn't show -- after a period or question mark.)