The Real Reason David Tennant Left Doctor Who
The year was 2008. David Tennant was coming off the most recent series of his highly acclaimed run as the Tenth Doctor on the modern revival of Doctor Who with Russell T. Davies as showrunner. Ten had traveled, lived, and loved Rose Tyler across every inch of the universe. The last four episodes stunned the sci-fi TV world with hit story after hit story, finally arriving at "Journey's End," which was (and still is) regarded as one of the finest episodes in all of Doctor Who's lengthy canon. The classically trained Shakespearean actor seemed totally unstoppable. Perhaps he was, save for the intervention of one person: himself.
That year's British National Television Awards program would prove to be a fateful one. Tennant was nominated — and ultimately won — for Best Drama Performance that year as the inimitable Tenth Doctor, but had been unable to attend in person. He accepted his award via video, and dropped an incredible bombshell that haunts many a Whovian even this many years on: He declared that the final collection of Doctor Who episodes to come in 2009 would be his final outings as The Doctor.
Those episodes eventually became semi-standalone specials that aired across 2009 and 2010, but the core fact remained: Tennant was quitting with no warning whatsoever. "Why," so many asked, "why now?" Time has not healed the wound in the hearts of many Ten lovers, but the circumstances make more sense in retrospect.
Appreciating one's own legacy
When Tennant made his earth-shattering announcement, his explanation was firmly set in a kind of reluctant resolve: "It would be very easy to cling on to the TARDIS console forever and I fear that if I don't take a deep breath and make the decision to move on now, then I simply never will." That's a tough thing to admit to oneself, and even harder to break to a fanbase so die-hard that a years-long hiatus didn't slow them down.
With the benefit of hindsight now, however, we can appreciate his logic; the fourth series of Doctor Who still stands out today as one of its best. All the companions within it are regarded as huge fan favorites, and most of their biggest adventures occurred within that series. That's unsustainable in television, as much as we wish it could be, and Tennant himself appears to have realized that.
Even if he were to stay in a role he loved, how much longer could the smash hits keep coming? Like all the best pop songs, Tennant got out at the exact moment everyone wanted more. Doctor Who is very important to him, so it's difficult to fault him for wanting to go out on a high note and avoid feeling stuck and unhappy in the role as the material inevitably declined. It shows an incredible amount of prescience on his part, though he did, according to Davies, "wobble" a bit, briefly reconsidering his choice in the aftermath of the announcement (via Digital Spy).
A lifelong fan
There's every reason to believe that, if Tennant had let himself, he would indeed have continued on for who knows how many series into perpetuity. The Doctor, however, must regenerate eventually, and Tennant decided that time should come on his own terms.
Like so many of his fellow Britons, he has been a Doctor Who fan his entire life, and being able to be The Doctor was a supreme childhood dream fulfilled. He married into a Doctor Who alumni family, for heaven's sake. Since his series-regular departure, he has been plenty happy to return for the odd special as well as the supplementary radio dramas. The love for Who and Whovians has never dulled. Even as he produced unrelated content in quarantine lockdown during the summer of 2020, fans noticed a to-scale replica of the TARDIS visible in his garden during a video (which he did not, in fact, take from the set in a cheeky bit of pilfering, in case you were wondering).
A ripe opportunity
The other less obvious factor in Tennant's choice to leave when he did was the opportunity to make a clean break. Series 4 would end up being the last full slate of episodes produced by Davies, who revealed in May of 2008 that he was stepping down to have Steven Moffat take over as showrunner (though Davies remained in charge of both Torchwood and The Sarah Jane Adventures). The set of individual specials released between 2009-10 were a joint project between Moffat and Davies to transition the screenwriting teams. At that time and place, it would have seemed like a relatively painless and easy point of disembarkation. Tennant got to have a coda for his iconic character, and the new showrunners got a clean slate to color on.
The choice was pretty clear: Leave now and let new blood get to make their stamp on Doctor Who's history, or commit to something far more expansive. It takes time for new showrunning to really find its stride, after all, and leaving in a way that seems premature can materially damage a program's reputation in the press. Tennant read the writing on the wall. He knew it was the best time to gain closure for Ten.
In the end, it panned out quite well. Tennant is still on call when needed, and Matt Smith (who became Eleventh Doctor) went on to be an extremely popular Doctor in his own right.