A covenant is not simply a contract. For one thing, it is unbreakable - not rendered void by the action of one party. This is an important teaching not only for couples about to marry in the Church, but something for us to consider whenever we're led into anger or resentment because of God's forgiveness or mercy towards others.
The vineyard owner's question to the laborers "Are you envious because I am generous?" is a good one for us too.