Greta Christina write approvingly on the value of forgiveness in Christianity. While I agree that forgiveness can be a useful psychological tool in many circumstances, I don't share her opinion that the specifically Christian concept of forgiveness is valuable: Indeed I consider it one of the most pernicious features of the religion.
Before we look into specifically forgiveness, we have to first look into the notion of moral conflict.
It is a scientific truth of psychology that people do what they choose to do: We can single out that behavior that is motivated by the mind (as opposed to falling, for instance, which proceeds without regard to one's mental state), and such behavior is motivated by the mind's propensity to effect what it evaluates as the best outcome by the best (or at least acceptable) means. To change someone's behavior, you have to physically change their mind, you have to physically change how they subjectively evaluate outcomes and means. (I'll write more later on the curious human habit of evaluating means.)
The notions of sincere shame, guilt and forgiveness (as opposed to faux "I'm sorry I got caught" shame) thus result from some sort of unresolved internal conflict. Someone who is unconflicted about what he or she thinks is best will simply do it. The only way that other people's opinions about what is best will influence someone's behavior is if part of their own internal evaluation of what is best includes other people's opinions and evaluations about some particular subject.
For instance, I don't feel a shred of guilt or shame about being an atheist, even though other people consider atheism to be, for one reason or another, really bad... even though as a normally socialized person I typically consider other people's feelings and opinions to be important. Since I don't internalize other people's opinions about my atheism, there's no conflict. It is only an internal conflict that generates shame and guilt; indeed it can said that shame is the awareness of this inner conflict.
(The most obvious and trivial instance of shame and guilt comes about because of a conflict between expectation and reality: A person will typically sincerely apologize if his well-intentioned action has an unwanted and unexpected result. If I step on your foot, I apologize to communicate that it was an accident, and you forgive me, perhaps with an admonition to be more careful. This sort of transitory social transaction is routine and unremarkable.)
Our internal moral evaluations come in two forms: intrinsic or "natural" morality, the specifics of which come from the general nature of our individual minds, and extrinsic or "artificial" morality, the specifics of which come from other people. (To get this extrinsic morality into our minds, there must be the intrinsic meta-morality that other people's morals are important; the differentiation is based on the source of the specifics. That I want to please other people in general does not itself entail any specific way of pleasing them.)
Evolution has rendered our intrinsic morality highly consistent. Except in unusual cases, internal moral conflicts are conflicts between intrinsic and extrinsic morality. Rationalization can resolve many of these conflicts without too many problems. For example, I have the intrinsic desire to simply take what I want. If there's something desirable in a box marked "FREE", I'll simply take it without a second thought. On the other hand, simply taking what I want from the grocery store violates specific extrinsically constructed moral notions about property rights. The conflict between the intrinsic and extrinsic morality causes a hypothetical sort of shame — I would feel ashamed if I were to shoplift — but because property rights are (at least for the most part under ordinary circumstances) rationally supportable, this hypothetical shame doesn't cause me any distress.
The resolution above really is a rationalization: the shame at the thought of stealing was directly indoctrinated into my mind by my parents and teachers; I didn't construct the evaluation by conscious deliberation. I apply rational analysis not to determine what to do or not do with other people's property, I apply rational analysis to resolve the conflict between my intrinsic and extrinsic morality. (Of course, it doesn't hurt that I've lived an entirely middle-class life with an enjoyable job: I can actually buy the things I want, and I've never been forced to choose between theft and starvation.)
Thus, feelings of guilt and shame become distressful and problematic only (under normal circumstances) when there is a conflict between an intrinsic and extrinsic evaluation which cannot be rationalized. Absent mental illness (a relatively unusual case), rationalization fails precisely because the extrinsic evaluation is indeed irrational.
There are two exemplary cases of these sorts of irrational extrinsic evaluations.
The most obvious is Christian sexual morality. In today's age of condoms and contraception, there is no good reason to choose lifelong monogamy with a single partner of the opposite sex if that arrangement does not suit your own desires and intrinsic morality. Furthermore, sexual desire is not only powerful, sexual orientation and preferences vary widely between individuals and appear to be highly stabile within an individual. A person, therefore, with an extrinsically constructed Christian sexual morality in conflict with her intrinsic sexual desire and morality cannot easily rationalize the conflict; the conflict will cause continuous guilt and shame.
Somewhat less obvious but, at least in some circumstances, even more outrageous is the extrinsic moral abhorrence of intrusive thoughts, again inculcated deliberately by Christianity. More outrageous because one cannot consciously control one's intrusive thoughts: It is no more rational to have a moral belief about intrusive thoughts than it is to condemn gravitational acceleration. Indeed it is this condemnation of the pre-conscious (coupled with "original sin") that underlies the Christian doctrine of inescapable human moral depravity.
Forgiveness is thus (outside of accident-transactions) a mechanism to moderate pervasive feelings of guilt and shame (and their attendant anxiety, distress and suffering) caused by unrationalizable moral conflicts, so that the person with the conflict doesn't go completely insane.
The rational solution is to not indoctrinate moral beliefs which are not at least rationalizable. In a perfect world, we shouldn't need any sort of "forgiveness" except in cases of accident and true mental illness. It's only to the extent that we allow parents to indoctrinate their children willy-nilly that we need some sort of social mechanism for forgiveness.
But the notion of Christian forgiveness is deeply pernicious. First of all, forgiveness is the primary virtue of the deity; to receive forgiveness, you must first feel the guilt and shame. And the guilt and shame has been inculcated by the completely irrational condemnation of intrusive thoughts and an arbitrarily restrictive morality. Christianity offers a palliative for a condition they explicitly created to make the palliative appealing.
That's not a blessing, that's a racket.