"Selfishly, I could have lied to myself and everyone else and said 'I'll tour', and I could have got on with it but I had to make a hard decision not to," Hartley, who was replaced by Owen Farrell as captain for the tour, told Sky Sports.
"The last time we played was here (Twickenham) against the Irish and it didn't go well for us. And you want to be part of the resurgence and what will be a successful tour for the team."
Hartley was sidelined for a significant spell two years ago due to head injuries and feared for his career. He had said that he would consider retirement if he suffered another concussion.
"Being here at Twickenham today actually makes you think: 'I'm not ready to finish playing yet', the feeling of being here and of knowing what the guys are about to go and do still excites me," Hartley added.
"Looking at who's touring, there's a good core of experience there but it's not the same England team. Owen will captain it, and he'll put his own stamp on that."
England's series in South Africa starts in Johannesburg on June 9 with matches on the subsequent Saturdays in Bloemfontein and Cape Town.
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by John O'Brien)
