Forgiveness and religion were bound together for many centuries  . Surviving shards can be found in anime usually in the mahou shoujo genre, where the heroine plays the role of the messiah and doesn’t simply banish evilness but most often purifies it by first of all forgiving the villain and then with words of love takes him/her on the good guys’ side. When I saw Araragi-kun in Bakemonogatari, I made that (faint) connection and got the urge to search the reasons behind his always forgiving attitude, since he wasn’t given any role to save the earth and there’s no actual reason for him to play the savior.
Koyomi Araragi was attacked by a vampire during spring break and became a vampire himself. Meme Oshino helped him become human again, but one tenth of him remains an ‘oddity’ or ‘aberration’. One may suggest that being an oddity himself helps him sympathize more with his attackers. During the series we see almost all the females possessed by spirits to attack him quite violently- Senjougahara thrusts into his mouth a stationary knife and a staple and she actually uses the staple on the inside of his cheek; Kanbaru punches him with murderous intentions only to see her later swaying him in the air from very strange places that cause nausea; Hanakawa has almost drained him of all his energy. Only Hachikuji and Sengoku don’t put his life in serious danger. After being through such adventures, it would be more natural to have at least an aversion or fear towards the supernatural.
Another guess would be that his self-healing vampire traits turned him into an aloof and foolish person (although I believe that’s somehow the case; he got overconfident). But that’s not it either. Senjougahara points out how Araragi would get involved, would be willing to help and sacrifice himself even without his powers. There’ s also Araragi’s understanding of how the world works- spirits just live they way they were made to live  and how everyone has moments of weakness or ill feelings. And of course, the fact that he gets himself involved willingly many times in unnecessary and masochistic extend, plays a huge role.
Surveys have concluded that there’s increased likelihood of forgiveness when someone is selfless empathic, his/her relationship with the perpetrator is one of importance, the offense is trivial, no responsibility is attributed and the perpetrator confesses his/her wrong-doing him/herself . I’ll add the factor of the distance from the hurt caused, the ability of erasing and forgetting events and if the other party is apologetic and seeks change.
In the case of Koyomi Araragi the offense is far more than trivial, the perpetrators (spirits) don’t confess their ‘sins’ and don’t seek change. That’s their nature after all. As for the responsibility of the people causing harm to Araragi or others (see Sengoku’s case), he seems to rationalize a bit too much their actions – in other words Araragi is selfless empathic. His character and worldview lead him in him being foolishly wisely  or wisely foolishly –whatever you choose- forgiving.
BUT in Nisemonogatari we hear the ever-forgiving boy say that there’s one person he can’t forgive: Shinobu Oshino or the former Kiss-shot-acerola-orion-heart-under-blade, the one who turned him into a vampire. Now isn’t that interesting? He says it clearly: ” I won’t forgive you and you won’t forgive me”; at the same time though he thinks the two of them have somehow reconciled. What is far more intriguing is how their little talk take places in the bathroom while both of them are naked, don’t get bothered from it and moreover hug (now this isn’t merely done for fanservice, is it? I think here can signify trust). I haven’t ever heard for such reconciliation without forgiveness. There isn’t also any trace of really bitter/revengful feelings. Should we say in this case that the relationship with the perpetrator is one of importance? Or is it a case where apologizing has already taken place? I’m reading Kizumonogatari to find out. What’s your opinion?
Intellectuals like Mahatma Gandhi and Oscar Wilde emphasized the superiority of forgiveness, too.
 Besides, trying to place the blame on the aberration isn’t right. They’re merely trying to exist, and that’s just what they are. Even aberrations have some semblance of reason for things they do.”