When should the personal a be used in a sentence, and why?

In Spanish, the personal "a" is used before a direct object that is a person or a pet to indicate that the action of the sentence is directed towards that specific individual. For example:

- Veo a mi amigo (I see my friend)
- Amo a mi perro (I love my dog)

The personal "a" is not used before direct objects that are not people or pets.