Why is life so difficult?

Why do people make other people suffer? One would think that if everyone gets hurt at least once, everyone would make an effort to never inflict pain on others again, having directly experienced how horrible pain is. We know this doesn't happen, so my only conclusion is that some people simply don't feel both the hurt they inflict and the hurt they receive because they have psychopathic attributes, such as the inability to feel and relate to human emotions. This, and instances when psychopaths hurt non-psyhopaths in such a way that spurns the non- into becoming destructive people like true psychopaths. As a result, we have cyclical, unlimited human suffering, as long as there are humans, or until genetic manipulation enables the elimination of undesirable traits like psychopathy (not suggesting I support that however).