If you were expecting some diet advice, sorry for disappointing you, this is not a dieting blog and advice is such a cheap commodity nowadays that it's practically worthless.
Bitterness is a... almost wrote metaphysical... state, the result of being bashed around the head with other people's best intentions.
One wants to relieve you from some innocuous agitation that has got nothing to do with them. You start by telling them about some upset and end up defending your reaction.
Moreover, you get some extra bashing disguised as genuine interest in your welfare. Resist, and the final blow will be administered as a suggestion to go and see a therapist.
Another one assesses your moods with clinical precision. Variations get quickly pushed into generalized statements. "You're always so and so on Mondays".
Really? Who could have thought that being as emotionally active as a corpse is the desired state. Everyone, apparently, everyone apart from you, the fickle, the moody, the over-reactive. Damn, time to bring in some bitterness and get a whiff of its exquisite bouquet.
At least, while being bitter there is no way anyone would try to push it any further. You're like that coffee that goes cold because there's no sugar left in the jar. Everyone knows sugar is not good for you. Right, there is a bit of dieting advice in here too.