The conservation of luck

"In the first of a new series, Ian Stewart, a professor of mathematics at Warwick University, takes a look at how we perceive luck and chance.

"A friend of mine won seven million on the Lottery," said the chap next to me in the gym. "That's the end of my chances. You can't win if you know someone who has."
There are as many urban myths about the National Lottery as there are legs on a millipede, but I'd not come across this one before. It set me wondering: why do people so readily believe this ...