I was never raised with pets, and my family sometimes treats me like dirt for caring about them. (ie. Volunteering at the shelter.) I am a little odd, though. I'll move worms off the driveway after it rains, and the one time my friend hit a possum I made him pull the car over so I could move it off the road and give it a chance. I seriously don't know where I learned it from, because everyone I knew as a kid weren't animal lovers. It's just one of those things that always seemed right. (When I was three, I couldn't pronounce my next-door neighbor's names - Bruce and Karen. It came out Boo and Kawin. But I say their cat's name just fine - Alex. My mother says now that it must have been a sign.) As for humans, I dunno. I guess I tend to care more than others do, but from personal experiance I don't trust and generally don't like to be around people.
I have friends who were raised in good homes with pets, and they seem to care less about animals. They reguard them as a chore, and pretty much ignore them. I can't understand why, because they have wonderful cats, dogs, ferrets, and birds. I tend to freak them out because whenever I'm over their places their animal is always on my lap. My friends don't seem to understand that I love it that way. They always ask if I'm being bothered by the pet, because they can "toss it outside" or put it in another room.
Even my mom is like this, "It's just a cat", and "Don't get attached to it."
And the reason I broke up with my last three boyfriends is because even though they were raised with pets, they liked to be mean towards my cats, because they though it was "cute" to see her get upset. I told them to stop, they wouldn't, so I kicked then out and told them not come back.
At the same time I've met the kids of other volunteers at the shelter (some older and some younger than me). They are all active in volunteering themselves, and love taking care of and braging about their cats and dogs at home. But their parents are active in animal rescue, so it makes sense that they learned about the importance of treating an animal right. They're also some of the friendliest kids I've ever met, and seem really comfortable around strangers, and aren't afraid to care about other's plights.
So I dunno. I think part of it is how you were raised, part of it is what you've experianced, and part of it is sheer luck.