Why does is my dog so aggressive and how to stop it?

My dog lexi is a 5 year old smooth haired chihuahua and she is so mean to everyone she don't know. She knows my mom, my grandparents, my brother, my aunt, and my brother's brother pootie and she won't bark or growls at them but if someone else comes around she barks, growls, snarls, and shows her teeth. She generally won't bite unless the stranger puts their hand near her mouth and it's very annoying when i walk her and she tries to pursue everyone and bark at them. Why does she do that? the only way i get her to stop is by spraying her with water but that works only temporarily. is their a way to keep make her permanently friendly.