Time Lords regenerate because they have a injury or something is wrong, it's their way
of healing themselves.
Trenzalore (his final resting place), in the town of Christmas
The eighth Doctor regenerated offscreen and nothing is known about how or why he regenerated. The eighth Doctor appeared onscreen only once in a TV movie. We know th Time War happened between the TV movie and the start of the 2005 series. It's widely assumed that whatever the Doctor did to end the Time War caused him to regenerate.
He regenerated because his current body was getting old, so it was of natural causes.
He died of old age, pretty much. He was an elderly man to begin with, and the stress and strain of defeating the Cybermen in 'The Tenth Planet' took its toll. He collapsed in the TARDIS and in a scene that no video footage remains for, he regenerates into the Second Doctor.
A Timelord can regenerate 12 times and have 13 bodies, the 13th time instead of regenerating he is supposed to die.
He will regenerate into the 12th Doctor.
He'll regenerate into the 13th Doctor.
The fourth Doctor fell off a bridge.
Eventually yes, he will regenerate into the twelfth Doctor.
Romana was another Time Lord, like the Doctor.
In the movie, The End Of Time.
Peter Davison.
It is unknown, the Eighth Doctor only appeared in a movie.
He would regenerate.
Trenzalore (his final resting place), in the town of Christmas
The current doctor (the tenth) will regenerate in the final special of 2009. Other than that there have been loads (old age, exile by Time Lords, radiation from cave, falling off radio telescope, etc ....
Because it was time for him to go he didn't want to be the doctor anymore and that's why they regenerate so that they can go from actor to actor without it being obvious