Why are dogs and cats the most popular pets?

I think they are generally easier to take care of. They definitely are very humanistic as well. You feel like they understand you (which I'm sure some do). Life span is definitely a factor. Our dog is 16 years old! It would be very depressing getting a pet to only have it die after a year or two. Some pets are not easy to interact with. Such as fish! I agree that you can easily bond with a cat or dog.