Bitterness

Can someone please tell me what bitterness is? Does it even really exist?

Are you comfortable calling someone bitter just because they have disagreed with the pulpit?

Could it be that only a small percentage of people labeled as bitter are just disappointed at how things turned out in their life?

Why isn't the thing or person that caused someone that disappointment and further, bitterness what is held in contempt opposed to the person being affected?