Repeated hurts increase sensitivity to said hurts. The more often a person hurts you, the less they must do to offend you.
Left unchecked, these hurts will grow and grow. A root of bitterness becomes an oak tree of offense. Distaste grows into disgust. In time, we take great offense at the slightest things. We recoil in pain from actions absent of ill intent. We may even begin looking for ways that this person hurts us, piling evidence on our case of how they are making our life terrible. I have walked this road, and its end is bitterness and sorrow. You will find no pleasure in preoccupying yourself with another person's sin. In fact, this causes the sin to multiply, and to spread from their heart into yours. Before long, you will drag other people into sin as you complain and criticize this foul person in a passive-aggressive coping mechanism. This is the danger of sin. It grows and multiplies, and one person's sin can spread into the hearts of many, destroying families, businesses, churches, and friendships. Do not do this. Leviticus 19 says, "Do not hate [your countryman] in your heart. Rebuke your neighbor frankly so you will not share in their guilt. Do not seek revenge or bear a grudge against anyone among your people, but love your neighbor as yourself."