[pontifier]

I'd say there are at least 2 morals.

First: You don't get what you don't ask for. the other person might really only want a quarter of the cake, but by asking for more than your "fair share" you open the door to allowing them to give you what you want.

Second: By asking for the whole thing, you have placed yourself in a position of power and privilege. The mere act of asking for more implies that you think you deserve it. Someone mentioned posturing in another comment.