Citation needed. People get bitter over good things sometimes too; because they see others as having gotten more, or perceive unfairness when there wasn’t any, and so on.
I am not implying bitterness is bad. But you can absolutely be bitter for almost any reason.
I have rarely encountered someone as wrong about anything as you are about this.
No it isn't. Most people only become bitter when they feel they have been treated unfairly and badly for a very long period of time.