As previously mentioned, each regeneration of the Doctor, while technically the same person with the same memories and the same beliefs, is functionally a brand new individual. The Ninth Doctor was an anxious man suffering from a severe case of PTSD. The Tenth Doctor was a warrior hero, brave and arrogant and dangerous. The Eleventh Doctor was a fool who refused personal responsibility for wit and whims. In comparison, the Twelfth Doctor is the weary old man who sits on his porch (which in this case is the totality of existence) and yells via Twitter and dental floss at passing youths.
But as with all previous regenerations, what is seen is not what is true. The Twelfth Doctor is just as frightened and lonely as ever, only now the exhaustion from his exams is written all over his face. He refuses to properly show emotion and verbally abuses Clara and Danny more often than anything, although he is aware that doing so will make them further away. In later seasons, Clara best describes him as, “You always worry. Always. Your reign of terror will end with the sight of the first crying child, and you know it.”
Anyway, that’s later. In season 8, Clara hardly trusts him because he rarely shows his inner goodness. In fairness, he last shows respect for Danny, whose Cyberman form (it’s complicated) ultimately saves humanity, but it literally took the poor guy dying for the Doctor to open up even that much. Hey, but small steps are still steps.