He felt that three years is the exact right time to play the Doctor for him and feels it's better to leave and be missed than overstay ones welcome.
He will fulfill his obligations to the upcoming Christmas special titled The Next Doctor and four more specials coming up in 2009 but says that he will no longer be the doctor in 2010.
Sad news indeed as he was the most popular Doctor to date and even I enjoyed his portrayal.
He made this announcement after receiving the outstanding drama performance prize at the National Television Awards.
Now the question is, who will be the next Doctor? Who will dare try to fill Tennant's shoes? The actors to come up most often out there might be Paterson Joseph or David Morrisey.
Regardless of who plays The Doctor, the executive producer, Russell T Davies, said they're planning a big ending, like dropping a radioactive piano on his head or something like that. Sounds like some nice dry humor going on there. I'd be careful if I were Tennant!!
Sigh... Why does nothing really good ever last for us, the Sci-Fi audience.
Source: BBC News