All emotions, whether they are positive or negative, have influences on relationships. Negative emotions, in particular, are important factors that can destroy relationships. Anger is one of those negative emotions that can ruin a relationship in silence.
Festering, boiling anger that stays within one's self is dangerous both to the person who is angry and to any relationships that that person has. It may seem as if a person's anger only means that that person is losing out on life, but in reality, anger is just like depression. It hurts the person on the inside, and it hurts everyone who is involved with that person.
Anger, like any negative emotion, is slow to build up. One small thing can lead to another, and often, when people are not willing to talk about their problems, the anger can build to such greatness that it becomes the sole reason why people cannot effectively have relationships with each other. Have you ever met an angry person? Have you ever been angry yourself? How did you feel towards others when you were angry? Were you able to communicate your feelings in a positive way?
Unfortunately, most people aren't able to communicate positively when they are angry. They can bottle their anger inside, and once they do decide to talk about it, the anger can be too much for them to talk about it in a civil and respectful manner. So, the problem with anger is that people often allow it to build up inside, whether it is due to lack of words to express that anger or it is due to the amount of anger being felt.
If you feel anger towards anyone you have a relationship with, whether it's a friend or a family member or a lover, you should talk about your anger in an adult manner. This is the only way to hopefully allow that anger to disappear or to quell it enough for you to seriously think about the condition of your relationship (whether you can completely take away that anger or not).
Like jealousy, depression, sadness, need for control, anger is an emotion that requires much time to talk about. If it is left in the dark, where it is allowed to build and grow to unwieldy proportions proportions that may someday destroy a relationship it can be dangerous not to just the angry person but to everyone with whom that person is connected.