Why does the Doctor regenerate?

Terms and Conditions Apply

As wondrous an ability regeneration is, it isn’t without its limitations. To start with there is a, well, limit. A regenerative ‘cycle’ consists of twelve regenerations, after which Time Lords are unable to regenerate. This means that the thirteenth incarnation of any Time Lord should be their last. Yet, there’s some wriggle room in that, hence the ‘should.’

You see, Time Lords can be given a whole new regenerative cycle. The process of how this happens is a bit vague, but it’s what allowed the Doctor to take out an entire Dalek fleet and live to tell the tale. 

What’s even handier about this miraculous ability, is that after regenerating, there’s a fifteen-hour period where the Time Lord’s cells are still settling. Within this window, their body can heal itself, without an issue, from all manner of injuries. It’s how River Song (assassin, archaeologist, owner of fabulous hair) managed to shrug off being shot as she was still within this time window. It even allowed the Doctor to completely regrow his hand! 
 

There is one major flaw with regeneration. It helps you survive almost anything, but if someone kills you in the middle of the process, then it’s game over. The whole process is aborted and the Time Lord is killed. This is what happened to the Doctor on the shores of Lake Silencio. Well, it… I mean… Spoilers.
 

Time Lords are able to exert some degree of control over the process, as they can opt to not regenerate. In the case of the Master, he flat-out refused to, just to be spared having to live with the Doctor. Later, the Doctor too didn’t want to change, exhausted at constantly being forced to become someone new.  

Luckily for all of us, he had a few words with himself - literally - and decided that more life couldn’t hurt.

The Truth Revealed

For many years, it was believed that regeneration was an innate Time Lord ability. However, after doing a bit of digging, the Master uncovered the truth. Being the Master, this naturally led to him first killing lots of people, and then confronting the Doctor. 

Regeneration was stolen from the Doctor as a child, who wasn’t actually originally from Gallifrey. Where they’re actually from, we don’t know. All that’s for certain is that they were found by the Gallifreyan explorer Tecteun, who experimented on the child Doctor. She discovered the secrets of regeneration, spreading them around the Gallifreyans. The result of this? The creation of Time Lord society, where regenerations were capped at twelve per person. Since this cap was imposed, it would explain how the Time Lords were able to grant individuals new regenerative cycles.  

What Next?

As for the next regeneration, we'll find out soon enough in the Thirteenth Doctor's final adventure, coming soon...