That's actually my favourite out of all her unreleased tracks. Doesn't make me mean, I read it in a different way. Sometimes people are never happy with what they've got, and the grass is always greener on the other side. Although, it's quite likely that once you get on the other side you realise that what you had before was better.
Ok, think about this scenario. You are Britney. Kevin left you for another woman over a year ago. You see him out with her on a regular basis. Despite that, he attacks some guy for acknowledging your existence because he considers you to still be his property. You feel desperately sorry for the other girl, because she obviously isn't making him happy, and you know that when he isn't happy, he makes everyone around him feel worse than he does.
Anyway, that's what that song means to me. I even made a video for it!!!