Until they realize that, such people misconstrue motives and don't wait to hear whether a partner's hurtful actions were inadvertent. They project.
"A clue to bitterness is someone who becomes critical," Wallace said. "Hold up the mirror and you may see the things that you're bitter about are things you haven't mastered in yourself, so you react when you see them on the outside. You attack, accuse, blame or complain."
Looking for solutions
Reading a book or starting therapy can smooth the romantic road, Wallace said. Even when people haven't resolved all of their bitter feelings, being open with a partner can work wonders.
"Let's say someone gets a divorce," Wallace said. "If they get serious with a new partner, they may want to say, 'I wouldn't want to lay these old feelings on you, but I might, and I'm sorry if I hit you with my old stuff. Anytime we start heading down that road, let's sit down and talk about it.' Of course, that's the erudite person who will say that. A lot of times they'll slam each other with it instead."
Bitterness sometimes rears its head on a first date, but more typically, Wallace said, it surfaces later, after the honeymoon period of a relationship.
That's a moment of reckoning for the bitter person's partner, Chen said.
"Ask the person, 'Do you see that part of yourself and do you want to change?' If they do, maybe you continue. We go on a bike ride together, and we have fun, because you ride your bike and I ride mine. If you want me to always carry you while I'm riding, this relationship will not work out."
Lancaster proposes a statute of limitations on breakup-related bitterness.
"It can't last longer than the actual relationship," she said. "I have one friend who got broken up with four years ago after six months, and she is still angry. We all talk about her behind her back.
"We'll be going out to dinner and ask her, 'Do you want to come out with us?' She'll say, 'Sure, I'll come out by myself because I don't have anyone.' OK, that was cute, but now it's bumming us out. And no one wants to set you up, because we like our friends. Move on."
Tips from the other side
To avoid wallowing in bitterness, author Jen Lancaster, author of "Bitter is the New Black," offers this advice:
1. Pursue something productive. "If you're creating something, you're not going to have time to stew."
2. Be among people, "even if you're exuding bitterness. At some point you will rein it in just to be situationally appropriate." But hold off on dating till your anger subsides.
3. Don't talk about it on the first date. "But it is something you share, because they need to know what you'll be like in a crisis."
4. Don't vent on the Internet: "The Internet is forever." Instead ...
5. Seek solace in friends. "And when your good friends get tired of it, it's time to give it up."