Regeneration into his sixth was more difficult than his last. Worrying that he might not turn out alright slightly damaged his psyche. It made him a bit unstable and more prone to violence. In his delirium he almost strangled Peri to death. Peri was convinced that he was unstable but the Doctor wouldn't admit he was. He was stubborn and egotistical. One or both of those traits would carry over from regeneration to regeneration.
Peri called his fifth regeneration 'sweet'. That wasn't him now. Being sweet made his too soft. He had to keep a firm hand of authority to be taken seriously. And he had an imagine to maintain. He wasn't slightly thin, not fat either, just a bigger structure to work with.
He had never had finer looks. Studying his reflection he saw a pure presence of authority. He didn't need that 'reckless charm' his
At one point in his adventures, a companion of a con man posing as the Doctor, (Sally-Anne her name was) had called him sexy. He found himself quite flattered by it. He spent most of his lives trying to flatter people, he liked it better when they were trying to flatter him.
The aliens invading the planet he and Mel were trying to save, wanted to imprison the Doctor. The Doctor fooled them into thinking the imposter was the real Doctor, by actual the Doctor kissing Sally-Anne. The one thing the Doctor would never do, kiss a girl. Not because he didn't like them. It was because he never understood the concept in the first place.
It was the first time he actually kissed a human being. Of course that didn't change his mind about humans. He still put himself above them but he still always did what was right. He didn't care what others thought of him as long as he was happy with himself.
After he was put on trial, facing the Valyard (the manifestation of his dark side, who planned to steal the last of his regenerations and the Time Lords having killed Peri, he went into retirement to avoid becoming the Valyard. And also to avoid meeting Mel. He was eventually lured back into traveling again. Eventually, despite his efforts he ran into Mel, whatever reason he couldn't out run their meeting.
It had been the first time he ran away from something he didn't want in his life.
If he couldn't avoid Mel, that meant he couldn't escape his inevitable fate.
