The Scottish actor played the lead role in the BBC sci-fi drama from 2005 to 2010.
"Doctor Who opened more doors than it closed," he told the Radio Times. "I was never bored, but I wanted to make sure I left before it became [just] a job."
Tennant - who will next be seen in BBC One drama True Love - added that he is still "very glad" to have starred as the Tenth Doctor.
"Mercifully, I haven't been typecast," he said.
The 41-year-old star recently advised new Doctor Who companion actress Jenna-Louise Coleman to "keep [her] nose clean" and "watch [her] back".
"You're suddenly rocketed into a world of attention," he told Digital Spy. "You just need to watch your back a little bit, but it's wonderful and it's such an exciting thing to be starting out on."
True Love begins on Sunday (June 17) at 10.25pm on BBC One.
> Doctor Who: Matt Smith and Jenna-Louise Coleman in new photo
Watch a trailer for David Tennant's True Love below: