The First Doctor often came off as grumpy and egocentric to strangers and new recruits. However, he had a big heart, which he revealed from time to time. His love for adventure stemmed from his necessity to be away from Gallifrey, but also from his restless nature and love for thrills.
The Second Doctor was younger than his predecessor, and this showed. He’d often play a tune on his recorder, or crack jokes with his companions. He often looked and acted sloppier than the First Doctor, and seemed less sure of himself and his actions. But his charm and positivity always won out in the end.
The Third Doctor, unlike the Second, was more of a man of action. He kept himself looking put together at all times, and resorted to logical means when it came to sorting out a problem. While the First was the patriarch, and the Second like a friend, Three was a mentor and teacher to his companions.
The Fourth Doctor rid himself of his preppy behaviors in exchange for bright colors and eccentricity. But his contagious fun-loving attitude often gave way to a deeper sadness, which made him even more feared in the eyes of his enemies.
And then there were five. The Fifth Doctor had a rocky start, which attributed to his insecurity and vulnerability throughout his existence. Still, his love for peace never died, and he always appreciated the little things such as cricket and companionship.
Though he nearly died, the Fifth Doctor made it through his regeneration into Number Six. This came with consequences, though. While the Sixth Doctor still enjoyed adventure and fun, his past vulnerability had done a complete 180, making the Sixth Doctor one of the most egotistical and biting out of the bunch. He even went so far as to mock his companions, something which the other Doctors would rarely do, if ever.
Finally, the Doctor found a happy medium for his personality during his seventh regeneration. The Seventh Doctor was kind at heart, and a mentor to his companions (much like Three,) but could become a cunning game master at the drop of a dime.
The Eighth Doctor showed a romantic side during his one appearance in the TV movie. It’s not clear why he changed from an “older man” to someone much younger, but we can chalk it up to years of loneliness, perhaps. (It had been seven years! On Earth, I mean.)
With the Time War behind him, the Eighth Doctor became the Ninth Doctor, and brought along with him a deep sadness and guilt. As the only member left of his race, he often exhibited wild behavior, to the point where his actions seemed suicidal. By the end of his “life,” he seemed to have no problem with the idea of sacrificing himself.
Though his sadness followed him to his new regeneration, the Tenth Doctor was more boastful about himself, and loved to laugh, even in the face of danger. Unlike his previous incarnation, who was more of a solider, Ten loved all walks of life, and felt guilt if he had to hurt them (yes, even the Daleks!)
Eleven: Both a young and old man at the same time. Though he still showed the boyish side of his predecessor, he seemed more world-weary than the both of them, as he was self-aware of the centuries of deaths he’d caused. This made him both warm and cold-blooded at the same time, a characteristic which often confused and frustrated his companions.