Today in Sunday School (we go to Gospel Principles now that Dave is the Ward Mission Leader) we were talking about tithing and why it is so important that we pay our tithes happily and with a willing heart.
In this last General Conference, Elder Bednar spoke about having 'clean hands and a pure heart.' I think this is the answer to why we must give our tithes happily and willingly. And of course the idea that we are required to not only have 'clean hands' (righteous actions such as paying our tithing and forgiveness of sins) but also 'pure hearts' (the changing of our very nature and motives) applies to more than just tithing. It is in every aspect of our lives, it's the very point of our mortal lives. We are not here on this earth to just 'do the right thing' that was Satan's plan. We are here to better ourselves, to "change our very natures" and to fix our wills to the Lord's so that our souls become worthy to live in His presence. The essence of God's plan for us is that we must use this life to strive for BOTH clean hands AND pure hearts.