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Because everybody has to make his own accountings, and set his/her own priorities as to how to spend or not spend their hard earned money. And family members are most often the first to judge about how and why their family members spend theirs.

They are trying to reduce the plea for financial assistance from family members. Some family members will start telling you how to spend your money once they know how much you earn. You can hardly turn down the financial plea of assistance from your family members once they know how much you earn. Some would even help you calculate how much you should have left from your last check. It's good to help family members in need mostly when the need is an important one, but one also has to be able to save for the future. so keeping your earning a secret is the way to go