I would say that there is no higher reason for it, but I still hesitate to call it arbitrary, because that seems to insinuate there is no logical justification for it either, which I believe there is (as explained in my whole natural selection thing before.)

I would not say that my behavior is based on being weak of will though. All my decisions (everyone's decisions in fact) are based on what they believe is in their best interest. I believe that there is no real benefit for me to lose the capacity for guilt, so I do not attempt to do so. Were I actually to lose the capacity for guilt, I might think otherwise, but currently my psychological state is such that I believe it is in my best interest to be capable of feeling guilt.