The new (2005–present) series emphasizes the Doctor’s choice to travel with companions because he enjoys seeing the universe through his companions’ eyes. The Doctor takes great glee in showing them how beautiful and diverse the cosmos is, and we may mistake the Doctor's search for a sense of wonder as his only motivation. We forget that the Doctor’s journey began in fear—fear of going home, fear of never going home, fear of being discovered by the wrong people, fear of the endless dangers of traveling through time and space. Even so, this fear led him to taking on companions, and benefiting from companionship. And indeed, to this day, his companions often reflect some aspect or another of the original TARDIS crew, their presence as much a security blanket as it is a source of joy.
The Doctor and his granddaughter Susan ran away from Gallifrey. The exact reasons why are still amongst the core mysteries of Doctor Who; we know that they refer to themselves as “exiles.” We learn much later that the Doctor not only stole his TARDIS in their flight from Gallifrey but also an astral engineering device known as the Hand of Omega. The Doctor possibly objected to something the Time Lords were going to do--or perhaps to their refusal to do something. Whatever the reason, the Doctor and Susan cannot go home; they are afraid of going home. The Doctor is also afraid of being discovered—revealing Time Lord technology to less advanced societies could expose people to things they are not ready for, and could expose him to the people he and Susan are running away from. The Doctor is afraid of harm coming to Susan, likely the only living family he has (the Second Doctor in “Tomb of the Cybermen” suggests his family is “sleeping in his mind”—in other words, they exist only in memories he dares call on only occasionally). And thus, the Doctor is in fact afraid of Ian Chesterton and Barbara Wright when they turn up looking for Susan in the Totter’s Lane junkyard.
Some fans, especially newer fans, are critical of how nasty the Doctor is to Ian and Barbara in Doctor Who’s first story, “An Unearthly Child.” Used to an outwardly friendly Doctor who loves humanity, these fans are perplexed by a Doctor who does not trust and is even verbally abusive to human beings. One must absolutely bear in mind two things: first, the Doctor you see in later stories had to get that way somehow; the beginning of the story shows the start of that process, and trust is not only usually earned, but also learned. Second, the story clearly sets up Ian and Barbara as the heroes—after the pan shot of the junkyard to establish its mystery, Ian and Barbara are introduced, and their personalities, interests, and concern for Susan are what drives them and the story; it is their quest to help Susan that is the plot. The Doctor is not the protagonist; he is in fact, technically, the show’s first antagonist, because he is the obstacle Ian and Barbara must overcome. Ian and Barbara's eventual triumph is that he joins the protagonists’ side. The show is called “Doctor Who” because it is the mystery of this “Doctor” which causes the heroes Ian and Barbara to get into the adventures they get; he is a driving force, a focus. Not till later does the Doctor also become the primary hero (in my opinion, however, that the best Doctor Who stories are those where TARDIS team, as an ensemble, are the protagonists, not where the Doctor alone is set up as the sole hero and the companions are the plot devices).
Most importantly, though, you have to accept and realize that the reason the Doctor is being a jerk is because he’s scared out of his mind. Look at it from his point of view: two adults have followed his teen granddaughter home. This alone is a little creepy, and he does not truly know why they have followed Susan (he does not know they are her teachers until later in the scene). He is trying to keep them from entering his ship, the knowledge of which he is afraid will cause them to contact Earth's authorities. For all he knows, they may be scientists or government officials tracking down his device (such as the proto-UNIT-like organization seen in “Remembrance of the Daleks,” which takes place in Earth chronology a few days after “An Unearthly Child”). He doesn’t know Earth or humanity very well yet. Susan is still the new girl in school; she has been at best there a few weeks, maybe months, and she has done most of the interacting with other human beings. Not to mentions, we humans have often proven to ourselves, let alone the universe, that we have a very ugly dark side. His goal isn’t to be mean to Ian and Barbara for no reason; his goal is to protect Susan and the TARDIS and from those who would fail to understand them and might hurt them or misuse knowledge of the TARDIS’s existence for their own gain. He takes off so Ian and Barbara won’t tell anyone about him, Susan, or the TARDIS. He is too frightened to take the chance that they would just leave it alone.
Soon enough they find themselves 100,000 years or so in the past, and at the mercies of the tribe of Gum. He realizes that first, Ian and Barbara are capable, and second, that Ian and Barbara have a vested interest in keeping each other and Susan safe. They are not selfish, and they are allied with his family. He comes to the right conclusion: he needs to stop bickering with them, and start using his incredible knowledge to help them. Barbara questions him—he had been irritatingly irascible until then, and he explains his helpful actions honestly with the quote above. He will work with them, because he is afraid not to. Fondness, respect come later, but soon. His fear forces him to work with them—and then he sees what they can do. Earlier he fights with Ian about who is “leader” of their group; later, the Doctor elects Ian to the position—and while the Doctor probably sees himself as head of their team in truth, he realizes Ian better serves as their spokesperson under the circumstances.
The Doctor turns to Ian and Barbara as allies because they help protect him. They do things he cannot—he may be brilliant, but he does not have Ian and Barbara’s empathy or at least their willingness to rely upon it as a benefit rather than a curse. In their third adventure, “Edge of Destruction,” Barbara accuses him of lacking both gratitude and common sense. The tirade takes him aback—he only then realizes how much they have contributed to their survival over the course of their adventures. Barbara is the one who shows him the TARDIS’s telepathic capabilities—he wasn’t aware of their extent until she deciphers the “message” the TARDIS was trying to tell them. So learning to see things through others’ eyes is a good thing, yes—suddenly, a dormant sympathy awakens in him.
BARBARA: "What do you care what I think or feel?"
DOCTOR: "As we learn about each other, so we learn about ourselves."
DOCTOR: "As we learn about each other, so we learn about ourselves."
The Doctor apologizes for his behavior. From “An Unearthly Child” through “Edge of Destruction,” we actually see one of the best emotional journeys the Doctor ever goes on, in any of his adventures—one where he learns to stop being afraid of companionship. The Doctor’s journey to trust is one that is relatively slow, but is appropriate, and all the more valuable for its subtle but profound effect on the stories that follow.
At the same time, the journey had to begin with fear. He never would have opened up to them were it not his own fear of them—leading to their capture—and fear of being without them—fear of death at the hands of mutual enemies. And he realizes, traveling with the people who become his friends makes thoughts of exile less cold and dark and frightening. His brief encounter with the “Meddling Monk” notwithstanding, the Doctor learns to forget about Gallifrey for a very long time, not until they capture him much later in the final Second Doctor story, “The War Games.” By then, he is less afraid and more outraged of "home" asserting its existence—the only fear there ultimately, is of Zoe and Jamie forgetting him.
The Doctor is still afraid. Now in a later part of his journey, he is afraid of losing people as much as he is afraid of facing the universe without them. Such is the "curse" of learning to benefit from friendship. The Eleventh Doctor’s dance between traveling with Amy and Rory but trying to leave them home between adventures reflects this strange attempt at balancing this fear. But he travels with people not just because he enjoys their company, or even because he enjoys seeing their adventures through his friends’ eyes, but also because they protect him. Their insights and bravery have saved him as often as his amazing abilities allows him to protect and rescue them when they need it. And he does so, because he knows a universe without companions scarcely bears considering.
Unsurprisingly, nearly all the companions have traits originally found in Ian, Barbara, or Susan (many of the individual traits listed cross over between each other).
Ian: Bravery, strength, willingness to fight, rationality, scientific curiosity.
Barbara: Emotional bravery, emotional/social curiosity, kindness, and willingness to speak up against wrongs—especially when the Doctor is wrong.
Susan: Brilliance coupled with innocence, youthful stubbornness, an openness to learning about new people (the Doctor learned this from his own granddaughter before it became his own trait), has a youthfulness or vulnerability that sparks a protective instinct in the Doctor.
The Doctor needs all of these traits in his companions in some combination to balance out his own brilliance, arrogance, curiosity, and powerful sense of justice. So he has someone to bounce ideas off of, someone to teach, someone to protect, and someone to, in the words of one of Barbara’s successors, “stop him.”