Reading in Pairs

Have you ever noticed how busses arrive in threes but movie plots are only repeated in twos? There are always three things that go wrong, but you wait for the second shoe to drop? Three wishes anyone? But only two fall in love in a romance? What is it with those twos and threes?

Of course, the mathematician in me looks at all the fours and fives we ignore--perhaps the number's a little to high to call coincidence. And then there are all the singles--truly, don't most busses arrive roughly on t...