Doctor Who has had various companions who have chosen to travel with him for there own reason. Every Doctor has had companions, as many as 10(1st doctor, spread out through 4 season)
The Doctor’s lastest companion, Rose, is the first companion of the Doctors with a fully fleshes out life story that the audience gets to see from her debut story. She, unlike other companions, has not been cut off from her family or the world she knew.
"This is my companion, she travels with me"