The Doctor of Doctor Who is effectively immortal. On those rare occasions when he dies, he regenerates into a new form, often with a new personality. It's a fine time to switch actors, which is probably good because William Hartnell (who portrayed the First Doctor) would be 103 if he still lived.
Brian Rimmer, who once assembled a fine video showing the introductory theme to every Doctor Who series, has compiled the regeneration scenes of all eleven Doctors. The last one at 8:45 is just plain funny.