Cain and Abel

Because God always sees through the true intention. And Cain had a dark, jealous intention lurking inside his heart. Pretending to be happy and really grateful. But like Cain, we can never disguise our true feeling from God, the overseer and over hearer of everything.

Abel offered the best to God with all his heart while Cain gave to God just the ordinary ones with ill-feelings which means that it did not come from his heart. He was not sincere in giving to God. That's why in the New Testament, it is stated that giving should come from the heart. Give what you are willing to give, no tithes or 10% of what you earn.