Regeneration is subject to some measure of voluntary control. In "Destiny of the Daleks", Romana chose to regenerate, even though she had no need to do so. In "The Twin Dilemma", Azmael also chose to regenerate. (In his case, this was a way of committing suicide, as he had already used up all of his regenerations.) In "The End of Time", the Tenth Doctor chose to delay his regeneration until he had completed his "farewell tour" of past companions. The Master, in "Last of the Time Lords", just refused to allow his regeneration to start &, in "The End of Time", we learned that he had made alternative arrangements (for his resurrection in an identical body).