The Step Four resentment inventory invites me to examine why someone else's behaviour is affecting me. In most cases, it's really obvious, because their behaviour is getting in the way of some concrete plan I have, for what other people think of me (pride), for my own self-image (self-esteem), my ambitions, my security, etc.
In some cases, the other person is not actually interacting with me at all, yet I am still bothered. In almost all cases, the person is reminding me of some thwarted plan or failure on my part. There are thus two types of resentment: resentments which are directly triggered by someone thwarting my plan, design, or expectation, or someone who is a reminder of a failure experienced at some other time, in some other way.