Pets are pretty dang expensive. Potentially thousands of dollars a year in vet bills, and hundreds in food, just to start. I had friends when I was a kid who had pets die because their parents couldn't afford vet bills (not very responsible to have pets when you can't afford to take care of them, but you can't tell someone not to have a pet any more than you can tell them not to have a kid). Among the people I've known, pets are quite probably the biggest nonhuman financial stressor - bigger than cars.

Shelter pets are not for everyone either, although avoiding pet store puppymill pets is obviously desirable.