It’s easy to understand why we shouldn’t subtract from the ten commandments that God entrusted to Moses. For example, skipping the part about keeping the Sabbath when we’ve got a ton of work to do undermines the underlying purpose of this commandment. Keeping the Sabbath teaches us that our self-worth is not to be based on what we accomplish but on our relationship with God, which requires setting apart time to be with God.
But why would Moses caution that we shouldn’t add to the commandments? I can think of two reasons. First, adding a commandment such as “Thou shalt tithe 20% of your income to the Church” is more likely to feed the ego’s desire to appear more holy than everyone else than to be a true act of generosity. Second, by choosing to add an easy-to-fulfill commandment — such as, say, “Thou shalt volunteer at your church’s food pantry once a month” — one can avoid focusing on a commandment one finds more challenging, such as “Thou shalt not covet” (thy neighbor’s new iPhone, spouse, job, etc.).
St. Benedict was aware of these traps, for in Chapter 49 of his Rule, “The Observance of Lent,” he says that each monk should share his Lenten resolution with the Abbot so it may be done with his prayer and approval. Why? The monk may unconsciously be seeking glory or avoiding needed reformation in his life through his choice of what he will or will not do.
The psalms constantly proclaim God’s wisdom. Thus we should (1) trust that God carefully crafted the commandments to help us mature in our relationships with God and our fellow human beings, and (2) resist the lure of pride, which leads us to believe that we know best how to conduct ourselves. God covered what we need to do in ten commandments — no more, no less. As it says in psalm 86, “Teach me your ways, O Lord, that I may live according to your truth! Grant me purity of heart, so that I may honor you.”