It's important to forgive other people, otherwise you hold some form of resentment inside. Holding this resentment inside forms a defence of sorts, within you, meaning when faced with similar circumstances, you'll react from abase source of negativity.
This resentment means you'll face these circumstances with different people, judging them based on your past. This resentments also means you'll face different circumstances with the same people, judging them on the actions from the original circumstance.
Forgiving someone for acting in a way which has hurt you, upset you, embarrassed you, shamed you, blamed you, angered you etc. Remember; these emotions are YOURS. You have the sole ability to work through the attached emotions and by doing so, forgiving someone will become a whole lot easier.
But because we hold on to the hurt; the pain; the anger - it is impossible to forgive.