This is great.  Love how simply and directly you came to this insight. Our cheerfulness is the point, for that pleases God and frees us from ego.  And it takes no effort and therefore no boast. Wonderfully wrapped revelation!

To the usual or worldly way of thinking, any giving is a sacrifice and cannot be considered a virtue unless there is some personal cost as well as conditions, quid pro quo most often. Where is the giving if there is no loss or pain to some degree, thinketh the world? Then there is also, as you mentioned, giving out of duty, the “obligatory” tithe. But as it says in the Tao: Perfect kindness acts without thinking of kindness.  Or it is catchier in the lyrics song by Janis Joplin: It ain't nothin' if it ain't free, babe.

The cheerfulness, for me, is that delight in God and life, a deep down gratitude in and joy of being, that has everything needed for others in endless supply, the giving more like spillage from an overflowing cup.