[rmunn]

You don't have to get that complicated to look for the reason. It's the purring. I am just one data point and you'd need to find others, but I find cats that purr loudly much more appealing than cats whose purring is so quiet that it's hard to hear. I believe if you were to survey other people who keep cats as pets, you'd find most would agree with that.

[wrp]

When I was a kid, my family got a kitten and a puppy at the same time. There were no other domestic animals around, so the two had only each other for socialization. The dog apparently observed how much attention the cat got when it purred, so it developed a habit of making a strange guttural noise when it was petted. We figured it was trying to purr.