Why do chihuahuas act scared?

It depends on a lot. The genetics of the dog, the dog's environment, etc. If a chihuahua came from two parents prone to anxiety/ stress, it is likely the reaction is written in the genetic code. If this is the explanation, it is quite possible that selective breeding played a role in the skittishness of the breed. If the dog is often around large and scary objects, objects that produce loud noise, or abusive owners, the chihuahua's fear is likely a result of constant exposure to such conditions. Permanently afraid, if you will. There are many combinations of factors that lead to the disposition of a dog, and if your small breed seems incessantly frightened without provocation, it is likely a product of breeding.