My dad was from the West Side, but

my grandfather when he first arrived in the US in 1907, lived on the South Side for a few years with his brother (who stayed on the South Side). That decade was the glory years for both teams - the Cubs won the World Series the following year, only 108 years before the next time - and he became a Sox fan due to proximity after embracing baseball. By the time he moved to the West Side, the Cubs had moved from the West Side to the North Side.

And on the West Side, it was divided and of course as kids the Cub fans (including his two closest lifelong friends) would jab my dad mercilessly about being a Sox fan when they were terrible and the Cubs were good. Of course that's the age when it all gets solidified, as it was for me growing up in the western suburbs.

The Cardinals were secondary (he was always primarily a Notre Dame fan as any Catholic kid was then), but since they played in Comiskey Park they were part of the package.