"Obviously delighted I'm back in there," Defoe, who has scored 29 league goals for Sunderland since the start of 2015-16 season, told the club website (www.safc.com).
"If I am being totally honest, I thought I would have had a chance last season, finishing the third-highest English goalscorer behind (Harry) Kane and (Jamie) Vardy.
"But it wasn't meant to be and then I started to think: 'You know what? Maybe it's just not going to happen'. But I didn't really complain or anything."
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Defoe, who is one of three forwards selected by Southgate along with Manchester United's Marcus Rashford and Leicester City's Vardy, said he was looking forward to meeting up with the new-look side.
"It'll probably be a weird feeling, because it's been a long time," he added.
"It'll be a different squad compared to when I was there. I've always looked forward to playing for England. It's the pinnacle. You don't get any higher than that."
England play world champions Germany in a friendly in Dortmund next Wednesday and Lithuania at Wembley in the qualifier on March 26.
(Reporting by Hardik Vyas in Bengaluru; editing by Sudipto Ganguly)
