Why did the Diadochi (Alexander's generals and successors) turn on each other so quickly and easily?

Maybe I'm just being naive, but the way I see it, these were men who had grown up with Alexander. They were tutored around him, raised alongside him, went to war, fought alongside him and followed Alexander to the edge of the world (by Greek standards anyway). Really, if that didn't bind them together and give them a sense of brotherhood and comradeship, then I don't know what will.

So why, mere years after Alexander's death, were they tearing each other and his empire to pieces? Were they all consumed by the same power hungry tendencies? Was it the treachery of one that turned them all against each other?

Why were they so willing to forgo all bonds of friendship and brotherhood and lose all that they had built? Certainly, not all of his generals and companions were childhood friends, but you would think that there was a certain sense of loyalty amongst them.

I'm probably being very naive and romanticising the politics and relations between the diadochi, please feel free to correct me