Giving in secret means nobody will know and praise you for the good deed.  Announcing it to everyone means you are fishing for compliments and flattery to stoke your ego, making your good deed not for others but for yourself.  The good deed becomes self serving and therefore is not a good deed.  Whereas the opposite of nobody knowing the good deed you have done but yourself and God is sincere and pure and in accordance with the Scriptures about putting others first before self.