The law forbids a father from rewriting the birthright to favor the child of the wife he loves over the child of the wife he resents. Feeling cannot overrule what is owed. A man must honor the firstborn's claim even when his affections pull the other way.
This protects the weak from the tyranny of a parent's moods. Justice means giving people their due regardless of how you happen to feel about them on a given day. The temptation to bend the rules toward those we prefer never sleeps. A father who honors an obligation over his own preference teaches his whole household that fairness outranks favoritism.