| Hello,
I think that genes (i.e. personality traits) and the environment you were raised in have a lot to do with it. Here's my personal example:
My wife is very outgoing, was one of 8 kids, and will forgive you for anything.
I am somewhat introverted, was raised as an only child, and I am not very forgiving.
Now, I think her personality was dictated mostly by genes and her willingness to forgive became a necessity living in a house with many other people. On the other hand, I think my personality was also gene-related (my father was the same way) and I didn't have to forgive anyone but myself as an only child.... just my thoughts. | 
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