He shakes and Growls way too much! Help!

by Elizabeth
(Lex,KY)

My boyfriend recently moved in with me and brought his Jack Russell who is 7 years old. He is a total sweetheart to me. The only thing I have noticed about him that seems abnormal is that when my boyfriend yells or we get in a fight he shakes really bad and looks so scared. He also growls and shows his teeth ALOT when you try and pick him up, especially if you grab him under his front legs.
The reason this seems weird or odd to me is because I have a pug who is also 7 years old. He never shakes when I yell or if I am in a loud situation. He has also never ever growled at me. So I was wondering if this was a Jack Russell trait or if perhaps something bad has happened to this dog in the past that would lead him to act like this.
I hope you can help because I am worried that maybe this poor baby has had a bad experience in the past. I hope that is not the case but I don't know anything about this breed, only pugs! ;) But I do love him and want to help!
Thanks!