Why does my dog have food aggression?

I have a 6 yr old mix breed and a 4 yr old rat terrier.I've had both from 6 weeks old and both weigh around 30 pounds.The terrier has food aggression and growls and snaps over rawhides.He is also jealous of the other dog coming near me.He will growl and leave the chair he is in with me if the older dog comes near.The food aggression is with me and the older dog.They have never seen a hungry day in their life.