One of the challenges of having a Chihuahua is making sure they are socialized, unlike other dog breeds, Chihuahuas tend to be fiercely loyal only to their owner and being very aggressive to anyone else. One way to stop that behavior is bringing your Chihuahua with you to appropriate events. Most cities will have dog parks and even dog festivals to attend. That is a great way to get your Chi around other people and dogs. Another idea is taking him/her with you to a friends house. Of course make sure your friend is ok with this, and if they are, it is a great way to get your Chihuahua around other people in a safe controlled environment. Once you Chi is around other people, make sure to praise him for going up to and sniffing other people. Encourage him to be bold and take treats out of other people's hands. Praise him for not growling or snapping. This will let your Chi know its ok to be around other people and not be so protective.