Originally Posted by Blackhorse47:
“As a matter of interest is there any official explanation of why the doctor takes on a particular form when he regenerates? Who/what is generating the forms as it always appears to be a surprise to him?”
It's pretty much random for the Doctor, other Time Lords have more control over the process.
The form of the Third Doctor was chosen for him by the Time Lords as part of a punishment.
Romana chose to look exactly like some one else (after trying various different "bodies" on).
The Master said he wanted a younger body when regenerating but it's unknown if he left the rest to chance.
As to why the Doctor seems to have more trouble than other Time Lords when regenerating it may be down to the fact that he only does it at the point of death so can't concentrate enough to get things how he wants them (to be Ginger for example).
Another theory is that the Doctor being half human does not have the skill of true Gallifreyian Time Lords and thus can't choose his new form.