humans are selfish, horribly so.
Not to say that this isn't universally true to all creatures (as I do believe; the idolisation of the animal world as ever understanding and kind is severely malformed and sickeningly optimistic) on this Earth of ours, but humanity seems to take it to a whole other degree. Beyond the usual "we do what we do for either short or long term self gain", I feel there's one other perspective that must be analysed. When a person helps a person in something, the outcome achieved us benefit Hirt the Leyden