Solar systems have a “goldilocks zone” where planets capable of sustaining life forms similar to ours can form. Well, galaxies might also have a goldilocks zone where habitable solar systems can form, too. Not too hot, not too cold, not too near the spinning void at the center of the galaxy and not too far out along the turbulent edge. Really, the main factor seems to be where metallic elements are likely to be found – the kind of stuff that forms rocky planets, which can then get wet. Do that enough times, and you get Michelangelo and Confucius.
The NASA/Caltech image came from Wikimedia Commons.