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The tenth doctor regenerated because he absorbed all the radiation from a radiation chamber to save Wilf's (Donna Noble's grandfather's) life. Wilf knocked four times, so the Doctor knew it was time for him to die (because in effect the tenth doctor dies and the eleventh doctor is born).

Episode: The End of Time (part 2)

I know this all true but actually the doctor was a little bit scared of what will happen to him when he regenerates for the tenth time because he absorbed all the radiation in order to save dona's grandfather so noble family won't respect him as a enemy. The doctor was friendly but angry at the time but wanted to filled the prophecy so he knocked four times. ]

first time : stop the master plan to take over the universe

second time : help donna's grandfather

third time : visited all companions even know he injured from radition

four time : successfully regenerated into his eleventh incarnation with a few injuries but did as wuz predicted

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12y ago

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