of The Doctor, an eccentric alien who travels through time and space and always ends up walking into trouble wherever he goes. There are times when the Doctor is fatally wounded and he cheats death through regeneration. Hence all the actors who play him. I don't know how fans reacted in the old Doctor Who series but one thing I've noticed in the new series is that some fans don't necessarily like the change of actor. I know when David Tennant left there was a huge uproar. People claimed nobody else could ever be the Doctor. David Tennant was THE Doctor and if he was leaving fans wouldn't be watching it anymore.
Regeneration is part of who the Doctor is and I don't mind it. I've always seen regeneration as character development. Again, I'd need some perspective on the old series but I've noticed that the Doctor does tend to regenerate whenever there's been a major change in his personality. Christopher Eccleston's Doctor was fresh out of the Time War and a less eccentric version of the Doctor that older fans would have been used to seeing. He was damaged and very guilt ridden. By this point he has seen way too many people, including his own race, die so at the very beginning of the new series he does seem a bit anti-social. He's used to bad things happening. Yet towards the end of season one he saved many people and it gave him a sense of hope (see The Doctor Dances). He softens up with a new companion and seeing people survive. By the end of season one when he's starting to gain a sense of hope again he's taken in the time vortex to save Rose and sacrifice himself. Cue regeneration number one of the season. His personality has changed a lot since episode one and now he's physically changing. Enter David Tennant's Doctor.
This tenth Doctor still had a darker side but he was definitely a bit more upbeat and had a whole new perspective on life. He was very determined to save people, possibly to make up for past actions and thought it was still achievable. Unfortunately his determination to save people becomes an obsession as he starts to save people who should have died, part of a fixed point in time. As David Tennant's era went on his Doctor was once again seeing more people die, this time in his place. He had lost one companion forever, one had her memory wiped in order to save her life, and another decided to return home after one season. Loneliness was a huge part of the tenth Doctor's life and by the end of his era he was set on saving people and not seeing any more loss. It was this change in personality where we learnt David Tennant was leaving and Matt Smith took his place as the eleventh Doctor.
Again, as soon as Matt Smith's Doctor appeared he was a completely different person. Smith was this bubbly fresh of breath air. It was a fresh start for this Doctor. He was excitable, childish, and by this time every companion he has known, past and current has a special place in both his hearts. Also by now, he has set himself as a saviour, a dangerous enemy, and a companion's hero. He had a thousand names, a reputation and at one point being known like this had all his enemies work together in order to save the world from him. That really shook him up a little. Matt Smith's Doctor really explored who he was to the universe. And then thanks to Oswin Oswald he was erased from the Dalek's memory base. He also faked his death to remain hidden. By what is now the end of Matt Smith's era as the Doctor, he has turned from a cheerful explorer to hiding in the shadows, not wanting to be known. The mystery of who he is has never been focused on as much as is it now. Darker sides of the Doctor are appearing more and there's this question of who John Hurt's Doctor may be. All we know so far is if it is another incarnation it's one who broke everything the Doctor stands for. Just in time for another regeneration and a new actor to step in and take the personality from there. What will the twelfth (or thirteenth if rumours about Hurt being the ninth incarnation is true) be like in the wake of the eleventh Doctor's changes?
We'll always mourn the loss of a great Doctor but look forward to each regeneration. It symbolises a pivotal stage in the Doctor's life. :)