Theories on Why Chicago is Friendly

A common comment on Chicago is "wow! for a big city, the people are very friendly!" Of course you can also run into a very unfriendly person in Chicago as well. But as an overall generalization, Chicago is considered perhaps the most friendly big city in the world. Okay... well, at least in the United States... Anyway... do you have a theory (or theories) as to why Chicago is so dang friendly?>