He's been in some undefined relationship, and the other party has broken it off, claiming that it 'didn't really mean anything'.
He sarcastically agrees 'yes yes yes, it was totally meaningless' (it certainly wasn't to him), and then attempts to convince himself that he never cared all that much in order to lessen the pain of separation ('sour grapes', essentially).
Seems pretty clear to me--
He's been in some undefined relationship, and the other party has broken it off, claiming that it 'didn't really mean anything'.
He sarcastically agrees 'yes yes yes, it was totally meaningless' (it certainly wasn't to him), and then attempts to convince himself that he never cared all that much in order to lessen the pain of separation ('sour grapes', essentially).
Pretty heartbreaking :/