I think the visceral reaction to it, at least for me, is that it's usually said by the same type of people (usually women and "women," let's be honest) who would talk about their "Haters" and other narcissistic stuff. There's a certain self-importance or something to it and it's often said by someone who has made major fuckups but usually avoids taking ownership. You know, like someone will cheat on their boyfriend or make some ridiculously dumb statement and get called out, and in an act of defiance will be like "Whatever, I'm living my best life so everyone can talk they [sic] shit." Even though it's usually women saying it, I can completely imagine Ethan Ralph hollering that line out after taking a Xanax.
For what it's worth, I've seen "Living my best life" used plenty of times and it's mostly been by white women who fit the description that I mentioned; I don't know if it originated with black women, but it certainly isn't just them using it.
