Why are dogs and cats the most popular pets?

I think the reason is that they are the most trainable animals, especially dogs. They can be trained not to harm people and even help them sometimes. Also, there are also small dogs that are very convenient for people who live in apartments and small houses. And, as shallow as it may seem, some people probably get dogs and cats because they're good-looking and cute. That is shallow, yes, but that is a probable reason.