Bitter is a common invective hurled around at writers by others in the writing business. I've thought about why exactly its so popular, much like you hear liberals describe anything outside their cultural marxist viewpoint as 'vile' and/or 'noxious'. I believe its because, much like those two terms, bitter prejudices the reader into favoring whoever is throwing around the adjective in question.
"Oh em gee, I don't like people who are vile/noxious/bitter!" thinks the reader, "I better steer clear of that bad egg!"
As someone who has been referred to by all three, these arguments are all emotion, no substance. Bitter is no exception. Bitter makes it sound like you're the one with the problem, while the other person is perfectly reasonable. Again, tumblr/social justice warrior style arguing rears its head: post a sarcastic comment, snarky image, or blend of the two and that's all the argument you need. Facts? What the fuck is a fact? That sounds like you're trying to other me, bigot!
I see bitter used a lot by agents and editors to describe someone who they rejected. Specifically "I got a bitter comment today. (sarcasm follows)" In my particular case, I waited a year to hear back from an editor, and got a form reject. My response was along the lines of: "You sat on my manuscript for a year and sent me a form reject? Classy."
Apparently, expecting a professional to do their job makes one bitter. "But Jack!" you say, "don't you know editors are very busy people?" Yes, but so am I, and I don't have time for this "poor pity me" bullshit that so many publishing gatekeepers (agents/editors/etc) love to hold up as an excuse for doing their job poorly. Does this mean every agent query sent out needs a line by line critique? Hardly, but if you're going to sit on a manuscript for a year, a critique or commentary is the mark of a professional, something which is sorely lacking from the young adult side of the industry. More and more, it seems as if you don't have an 'in' with an agent or editor, you're not really someone they'll deal with. I'll talk more about the paradox in how publishing has become more closed as editors and agents have become more avaliable in another post.
That being said, I'd like to thank Eileen from WorldWeaver Press, who had my partial manuscript for ten months, and sent me back a nice two paragraph critique of what she liked and what she didn't. While she's working in a smaller imprint, her professionalism is appreciated.