3. Moral Values
To put it simply, if Doctor Who was continuously happy from start to finish, everyone would lose interest. Giving the story its much needed depth and darkness reflects the audience, too, as it highlights the moral dilemmas of each episode and the consequences they have for the characters involved. They're not always good ones but the fans are with them every step of the way. The audience become emotionally invested in the characters which is vital in order for them to be able to suspend their disbelief. Doctor Who is known for its somewhat exuberant plots and without that touch of realism, viewers would no doubt find it much harder to relate to the titular Time Lord and his foray of coveted companions. Without them and the real issues that affect them, it would basically be a show about a man stuck inside a box. If you like that sort of thing, you could always check out The Cube on ITV...