My dog is scared ?!?!?

My Jack Russell Terrier (my baby,a male) has always shook why? We have another dog who is his girlfriend :P We just got another dog,a female, who is an 11 week old mastiff. He shakes a lot around her but he growls at her and stuff. Now why did he and does he shake? Any ideas how I can make my mastiff Ises and my jack russell Rockey friends?