The striker was left out of Roy Hodgson's 23-man line-up for the upcoming tournament, but is one of seven players on standby.
Capped 55 times by his country, Defoe now plays for Major League Soccer outfit Toronto FC, having left Tottenham in the January transfer window, but he feels that he deserved to be heading to south America.
"I don't think I'll ever be at peace with the England decision," said the 31-year-old.
"If I had an injury, then I could sort of understand it. I could hold my hand up and say, 'it was not meant to be'.
"But in my head it's baffling a little bit. I don't really understand it.
"I don't think I need to prove to the England manager that I should be there. I think I've done that throughout my England career. I've always worked hard."
Only one of Defoe's 10 goals for Spurs last season prior to his move to the Canadian club came in the Premier League, but having been a regular part of the England set-up he is unhappy at losing out going to a World Cup for the second time.
"When you've been involved in every squad and then the World Cup comes around and you're not in the 23, it's hard to take," he added.