"Under what circumstances would you not forgive? "
Well, your original question was pretty broad. You didn't say: 'Should I always forgive someone who hurts me?' Or, 'should I always forgive someone for what they have done to me personally?
There are a number of situations where we are in a position of authority, where someone sins against another person (not directly related to us at all), and we are not called to forgive that person, we are called to reprimand and serve justice to that person. For a judge, that can mean physically having someone sentenced to death. I'm glad I don't have to make that kind of decision.
He who justifies the wicked and he who condemns the righteous are both alike an abomination to the LORD.
God is completely Just. Without that foundation of justice, there is no real meaning behind what Christ did on the Cross. He doesn't forgive everyone, lest there would not be a single human being ever thrown into the lake of fire. There will be though. That's scary.
If someone sins against me personally, and insofar as I am capable, I intend on forgiving them. Often times seeing God's justice served in a temporal sense can help make that forgiveness easier. But, even if I don't ever see that justice served, it's much better for me to forgive them than not to. At the end of the day, complete justice will be served to every single person who ever lived. Those who are covered with garments of righteousness, washed in the blood of the lamb, get exactly what they deserve too, but only through the grace of Jesus Christ (He took what we deserve, so that we can share in what he deserves). I don't like that bitterness and resentment welling up in my heart that unforgiveness brings with it. It kills love.