What are the "causes of death" of the Doctor?

Question

Each regeneration occurs because of something, like radiation poisoning from the 3rd to the 4th and 10th to the 11th ... or old age as to 11 to new cycle.

But, what are the causes of his "death" ? What are the reasons behind each regeneration?

Answer 1

If I understand what you are asking, this Doctor Who Wiki page will answer your question.

The relevant bit here:

• First: Claimed to be "wearing a bit thin"; apparently died of simple old age, but other accounts suggest he also had his waning life force pulled out of him by the planet Mondas.
• Second: Forced to change his appearance by Time Lord court order, and eventually executed by animated scarecrows. Sentence passed in The War Games, carried out in The Night Walkers
• Third: Radiation poisoning upon exposure to The Great One's highly unstable web of Metebelis crystals.
• Fourth: Fell to the ground from great height at the Pharos Project; indirectly caused by the Master.
• Fifth: Refusal to take antidote for spectrox toxaemia in order to give it instead to Peri Brown, succumbing to the fatal effects of the poisoning.
• Sixth: Ambiguous injury consequent to the Rani's attack on the TARDIS. Some accounts indicated the Doctor hit his head on the TARDIS console. Others suggest that he was also suffering from a chronal energy drain after his confrontation with the Lamprey and this served as the killing blow.
• Seventh: Following a non-fatal shooting, the Seventh Doctor underwent open heart surgery in a San Francisco hospital. Since the attending cardiologist had no knowledge of Time Lord physiology, she accidentally killed him.
• Eight: Died after a ship he was in crash-landed on Karn; temporarily resurrected to consume the Sisterhood of Karn's Elixir of Life and control his next regeneration for the Last Great Time War.
• Ninth [The 'War' Doctor]: As with the First Doctor, he claimed to be "wearing a bit thin" and regenerated from what appeared to simply be old age.
• Tenth (Commonly called Ninth): Directly absorbed time vortex energy from the Bad Wolf to save the life of Rose Tyler.
• Eleventh (Commonly called Tenth)¹: Shot by a Dalek, but routed excess regeneration energy into his own previously severed hand, resulting in no physical change, but using up a regeneration to survive the lethal attack. This 'spare' regeneration energy was used to create the Meta-Crisis Doctor from the severed hand with help from Donna Noble.
• Twelfth (Commonly still called Tenth, but technically a new version with the same face): Intentional radiation poisoning incurred in order to save the life of Wilfred Mott.
• Thirteenth (Commonly called Eleventh): As with the First and War Doctors, his body succumbed to old age, but by a far greater amount than either of them. This marked the beginning of a new regeneration cycle granted by the Time Lords.
• Fourteenth (Commonly called Twelfth): Sustained fatal injuries while fending off an army of Cybermen.

_{¹ -11th Doctor's details added by me; not listed on the linked page.}

Answer 2

Each time he changes, he doesn't die, he just heals himself, and his brain cells rearrange in such a way, that his personality (how much it changes varies) and likes and dislikes can change. Though he still has his core personality, having an intense hatred for the Daleks, and being a dark but kind man. Here's how he's regenerated:

• 1st: Was apparently growing weak from old age. He collapsed from exhaustion while battling the Cybermen on Mondas.

  • First Appearance: An Unearthly Child
  • Regenerated: The Tenth Planet

• 2nd: The Time Lords punished the Doctor for breaking their laws of non-interference by forcing him to regenerate into his next incarnation.

  • First Appearance: The Tenth Planet
  • Regenerated: The War Games/The Night Walkers

• 3rd: Died from the effects of extreme radiation poisoning caused by the Great One's web of Metebelis crystals. Fellow Time Lord K'anpo Rimpoche saved the Doctor's life by kick starting the regeneration process, which caused him to change into his 4th incarnation.

  • First Appearance: Spearhead from Space
  • Regenerated: Planet of the Spiders

• 4th: The Master indirectly caused the Doctor to fall from the top of the Pharos Project tower, causing him fatal damage. As he lay injured and dying, the Watcher (a being thought to represent something between the Doctor's 4th and 5th incarnations) merged with the Doctor and saved his life by transforming him into his next incarnation.

  • First Appearance: Robot
  • Regenerated: Acropolis

• 5th: After dropping the contents of the antidote for the pathogen, Spectrox Toxaemia, the Doctor saved Peri Brown by giving her the last vile. He then succumbed to the effects of the disease and regenerated.

  • First Appearance: Acropolis
  • Regenerated: The Caves of Androzani

• 6th: Unknown

  • First Appearance: The Caves of Androzani
  • Regenerated: Time and the Rani

• 7th: Somehow regenerated in a San Francisco hospital's morgue hours after surgeon Grace Holloway, accidentally killed him with anaesthetic and a probe while trying to fix his "heart problem".

  • First Appearance: Time and the Rani
  • Regenerated: Doctor Who (1996)

• 8th: Was killed when the spaceship he was on crashed landed on the Gallifreyan colonial planet called Karn. He was brought back from the brink of death by the Sisterhood of Karn, and regenerated into the War Doctor by drinking a specialised Elixir of Life, so he could fight in the Last Great Time War.

  • First Appearance: Doctor Who (1996)
  • Regenerated: The Night of the Doctor

• War Doctor: After centuries of fighting in the Time War and time locking Gallifrey along with his twelve other incarnations, the War Doctor regenerated after apparently "wearing a bit thin".

  • First Appearance: The Name of the Doctor
  • Regenerated: The Day of the Doctor

• 9th: Absorbed the energy of the Time Vortex from Rose in order to save her life. This caused rapid cellular decay, and he regenerated shortly after in order to save his own life.

  • First Appearance: Rose
  • Regenerated: The Parting of the Ways

• 10th (11th regeneration): Regenerated and healed his body after nearly dying from being grazed by a Dalek's laser beam. Avoided the change into what would have potentially been his 12th incarnation in the nick of time, by directing the transformative energy of the process into his spare hand.

• 10th (12th regeneration): The Doctor locked himself in a radiation booth and absorbed an amount of rads that would've instantly killed an ordinary man. Held back the process in order to bid a final farewell to his former companions. He eventually regenerated (which destroyed the TARDIS console room) into his 13th incarnation.

  • First Appearance: The Parting of the Ways
  • Regenerated: The Stolen Earth/Journey's End, and The End of Time: Part 2

• 11th: After a near millennia of protecting the people of Trenzalore from many of his old enemies, the Doctor was given a second regeneration cycle by the Time Lords, just as he gave up his life. He used the energy to utterly destroy the Daleks and their entire fleet before returning to the TARDIS in the form of a young man. With the process in its final phase, the Doctor accepted he was always going to be the same person deep down, no matter how many times he changed. He then became a new man a thirteenth time (but regenerated a 13th time), becoming the 12th Doctor.

  • First Appearance: The End of Time: Part 2
  • Regenerated: The Time of the Doctor