Indeed, the Tenth Doctor was described by the Moment as "the man who regrets".
The fact that he was instantly able to recall the number of Gallifreyan children that he believed had been killed during the war indicated just how keenly he actually felt those horrendous memories.
When traveling without companions, the weight of being the last Time Lord became much more pronounced.
If prodded too much, he would erupt in an almost righteous sense of fury.
(PROSE: Timewyrm: Revelation) After sealing Gallifrey away in a pocket dimension, (TV: The Day of the Doctor) the Seventh Doctor was able to recall teaming up with his other twelve incarnations to save Gallifrey.
(AUDIO: Cold Fusion) After using a Deathworm Morphant to posses a human body, the Master tried to use the Eye of Harmony to steal the Eighth Doctor's remaining regenerations to heal himself, but his plans were foiled when Grace Holloway sent the TARDIS' into a temporal orbit.
However, when he had to engage in conflict, the Tenth Doctor's energies would shift to reveal a fiercely protective man who resolutely guarded what he held dear, and who could be viciously unforgiving to the forces that dared menace them.
(TV: Utopia) After regenerating, the new Doctor noted his new teeth, (TV: The Parting of the Ways) and intended to take Rose to the planet Barcelona.He snapped out of it briefly to save Mickey and the Tylers from a Roboform Christmas tree, but the stress of waking up too soon made matters worse and he collapsed again.He slept through much of the ongoing invasion by the Sycorax.(COMIC: The Transformed) The Ninth Doctor absorbed the Time Vortex from his companion, Rose Tyler, to save her life when she became the Bad Wolf entity and transferred it back into the Heart of the TARDIS.
The forces of the time vortex began to destroy every cell in his body, and he had to regenerate to save his life.Many of his relationships with companions were shaped, to one degree or another, by romance; he seemed to genuinely love Rose Tyler as the person who helped heal some of the scars of the Time War, was completely oblivious to Martha Jones' obvious affections toward him, and insisted upon a platonic relationship with Donna Noble.