According to David McClelland, all people universally have three different types of needs which drive our behavior. These include the need for achievement, power, and affiliation. Everyone has all three needs, but some or one of these needs may be stronger than the others in a person.
One would say that Stephen Strange is driven mostly by his need for achievement. He became one of the world’s topmost neurosurgeons. The Ancient One, however, disagreed, indirectly, with McClelland's theory. She comments that Stephen is not so much driven by his need for success but by his fear of failure.
Throughout the movie, Stephen goes through three stages of personal development that I think is illustrated by what he calls himself. When Stephen first comes to Kamar-Taj, the Ancient One refers to him as “Mister Strange.” He quickly corrects her with a snarky “Doctor.” However, this arrogance disappears when Stephen meets Wong, who also refers to him as “mister.” This time Stephen humbly says “Stephen is fine.” What changes?
The third time someone refers to Stephen as “Mister Strange,” he again corrects them, this time back to “Doctor Strange.” He is no longer just a humble student, but the arrogance is still lacking. This time, Stephen owns his identity with self-assurance and confidence.