Because forgiveness is better for you than hate. Even From a egoistical point of view, only.
If it was your turn and somebody somehow hurried like crazy in front of you... it's his fight... maybe you can try to keep your turn anyway but without hate, just asserting yourself. Hate is self-destructive, maybe it can destruct the other... but sure the hater.
What is different is forgetting. I forgive but not forget (you have heard it before
). I mean I forgive someone, but when meeting again... I use to have it in mind what happened... that goes away with time, but people use to be repetitive and do the same things again... so maybe he/she would do a similar thing again (thought not so important), so there you are reminding again... and forgiving again...
We all should forgive, we all do mistakes, mainly coming from ignorance. "We just don't know what we do"