When I'm nice, it is because I am doing a favor, because I want to. It is not my default setting. It is not because "I'm nice." Niceness is not a trait - it is a personal gift.
I give a lot of gifts. All day, most days. To a lot of people. But they are still valuable. They are still conscious decisions. They are not owed anymore than a neatly wrapped game system under the Christmas tree is owed.
You may not choose your gifts and give me a shopping list - they need to come from me.
You may not get angry with me or punish me for not getting you enough gifts if I don't have the budget, or if you expected more.
The more you steal from me, and the less you say thank you,
The less I want to give you.