"I've had a pretty frustrating couple of years," the 31-year-old told Radio Sport on Wednesday.
"You have times where you're like, 'Is this really worth it?' or 'Is this going to come right?'"
Coles was named in Steve Hansen's squad for the Rugby Championship on Monday despite still recovering from ruptured knee ligaments suffered against France last November.
The injury, which required two operations, the second in May to remove scar tissue, came on the back of knee and calf issues and concussion symptoms that ruled him out of the series against the British and Irish Lions last year.
Coles is expected to be fit for the tail-end of the southern hemisphere competition, which starts on Aug. 18, but will play for Wellington in New Zealand's provincial championship beforehand to get match fit.
He added that the thought of getting back onto the field had kept his motivation from flagging and that he did not want to bring his 56-test career to an end because of injury.
"I think the drive, especially for me of getting back out there playing for the All Blacks or playing for the Hurricanes, has been a massive motivation," he added. "Just playing rugby again. I love the game.
"I didn't want the injury to determine my outcome I wanted, leaving on my own terms I suppose."
In Coles' absence, Canterbury Crusaders' Codie Taylor has seized his opportunity.
The 27-year-old has made 10 of his 13 starts in the number two jersey since Coles was injured and while he does not possess the pace of his Hurricanes counterpart he has the soft hands and passing skills required as an All Blacks tight forward.
Taylor has accumulated 32 test caps since he was a surprise choice as third-string hooker ahead of the 2015 Rugby World Cup and will give Hansen an experienced one-two punch in the role at next year's tournament in Japan.
(Reporting by Greg Stutchbury; Editing by Peter Rutherford)
