Jack Russell Terriers were "created" to run with a pack of Foxhounds and go down the hole after a fox. Because of this, they must be fearless fighters.
They need to dig and they need to have the stamina to run with a pack of Hounds. JRT's must also have the courage of a Lion to fight that fox, in a hole. They are instinct driven, as they should be. A Jack will run until it drops, and fight to the death, whether it be a raccoon, groundhog, or fox.
Many people are fooled by their adorable appearance and underestimate their needs. They also make good ratters, and will happily rid their home of any rodents or vermin. Unlike a cat, they will not always bring their prize to you, but will dispense with it entirely.
This is why they were created! They were not a recognized breed in North America until fairly recently. Being involved with JRT's (from England) since the early 70's, it is disheartening to me to see people now trying to make them into apartment dwellers or feisty lapdogs. To do that is to kill the very spirit of a Jack Russell. If one is not prepared to allow them to flourish in the manner for which they were intended, they should not own a Jack Russell, in my opinion.
They make excellent companions as they are quite portable, and get along well with humans. It is my sincere hope that people who acquire a Jack Russell, such as the OP, provide them with an environment full of stimulation that allows their nature to shine.