The striker has been recalled by the Three Lions for the first time since 2013 after scoring 14 Premier League goals this season, despite the Black Cats battling relegation.
Defoe could feature in Wednesday's friendly in Germany ahead of Sunday's World Cup qualifier against Lithuania at Wembley.
He wants to make Gareth Southgate's squad for next year's tournament in Russia - should England qualify - but conceded that would be in doubt if Sunderland were relegated.
"You have to be realistic, to get into any squad is based on merit and to do that you have to be playing at the top level," said Defoe, who has scored 19 goals in 55 games for England.
"When I went to Toronto (in 2014) I missed out on the World Cup. When I got the call it was hard to accept but you had to be realistic and think the boys I was competing against were playing the Premier League and Europe and that's why I missed out.
"So, yeah, I would say you would need to be playing in the Premier League but we'll see what happens.
"I know how important it is to play for England. Just going to a major tournament, travelling, getting to the hotels, playing the games - it's something you've always wanted to do from being a kid, growing up watching the great players over the years.
"I've been lucky enough to have experienced it. If I do get an opportunity to get to another major tournament it will be a dream."