The naive striker asked to leave the club in 2003 – just 24 hours after The Hammers were relegated to the old Division One.
It still haunts Defoe, who signed for West Ham at 16, and it is a huge factor behind attempts to get him to go back.
A source close to the player said: “Jermain still feels terrible about the whole thing. He was badly advised at the time, when he was just a kid. He accepts that.
“He loves Sunderland and is doing his best for them and will continue to do so. But there is an emotional attachment to West Ham.
“If a deal is done and Jermain gets the chance to go back there and score the goals which make sure they stay up and move up the league it would help him heal some old wounds which still hurt him.”
Defoe has been labelled ‘Judas’ by angry Hammers supporters ever since leaving them to join arch-rivals Tottenham.
West Ham are preparing a second bid of around £6million for Defoe, 34, who Sunderland say is not for sale during the transfer window.
He underlined his importance to them by scoring twice on Monday to rescue a point in a 2-2 draw with Liverpool.