I did my post-doctoral training in a very unusual place: Skid Row in Los Angeles. I trained at a center that rehabilitated the homeless. It was actually a wonderful program for those who were homeless, as they could also receive intensive mental health treatment.
As part of my training, I co-led groups with clients recovering from addiction. I learned more in that year than I may have learned in all the years of graduate school!
The 12 steps (first, admit you have problem) form the cornerstone of most clinical work with addiction. One of the things I learned from the 12 step literature is the difference between anger and resentment.
Anger is an emotion you feel in the moment, while resentment is the accumulation of anger that has gone unexpressed. Anger is good but resentment is bad. The idea is that anger must be expressed in order to get over it. Many addicts turn to substances to self-medicate as some of the feelings they feel are difficult to sit with or accept. Later in recovery, they find that there is often a great deal of anger they must face in order to heal. Yet addicts aren't the only ones who have an anger buildup. Unexpressed anger can infiltrate any relationship and damage it.
In my practice, I see that clients often hate anger. They are uncomfortable with it and think of it is bad or ugly, an emotion that is unhealthy to feel. This is the precise reason why people don't like to express it and why it ultimately turns into resentment.
Trust me when I say anger is a lot better for a relationship than resentment. Resentment can really destroy the fabric of a relationship, so make sure you express the things you are angry about in your relationship.