The basis for the ethics of the Sermon on the Mount is not what works but rather the way God is. Cheek-turning is not advocated as what works (it usually does not), but advocated because this is the way God is - God is kind to the ungrateful and the selfish. This is not a stratagem for getting what we want but the only manner of life available, now that, in Jesus, we have seen what God wants. We seek reconciliation with the neighbor, not because we feel so much better afterward, but because reconciliation is what God is doing in the world through Christ.