Marriage is a "covenant or partnership of life between a man and woman, which is ordered to the well-being of the spouses and to the procreation and upbringing of children. When validly contracted between two baptized people, marriage is a sacrament" (Catechism of the Catholic Church, 1601).
The last sacrament to be considered is the only one of the seven sacred mysteries to be explicitly called such in the New Testament: Holy Matrimony. Sacred Scripture begins with the creation of man and woman as spouses in the image and likeness of God and concludes with a glorious vision of the Wedding Feast of the Lamb. In other words, the whole of creation and redemption is a marriage between God and His people, and for this reason St. Paul teaches that the union of husband and wife is an image or icon of the union between Christ and His Church (Eph 5:25-32).
St. John tells us that the first miracle worked by the Lord Jesus was at a wedding feast (John 2:1-11), thus revealing the intimate connection between the messianic mission of Christ and the dignity of marriage, which is the one blessing of God not lost by original sin or washed away in the flood. The bond of husband and wife, however, was disfigured by sin (like everything else in nature), and requires the grace of Christ to be purified, healed, and restored to its original dignity.
Because of the weakness of human nature after sin, Moses permitted the children of Israel to repudiate the solemn covenant of marriage, but the Lord Jesus restored the original pattern of spousal love by revealing that the true bond of marriage, once begun by the mutual consent of the spouses, can never be broken in this life.
By restoring the original order of creation, Jesus was not giving us a burden too heavy to carry, because He Himself gives the strength and grace to live marriage as a permanent union by following Him in the Way of the Cross, renouncing oneself, and living for the sake of the other. The grace of sacramental marriage is a fruit of Christ's holy Cross, the source of all Christian life.
Unlike the other six sacraments which are all administered by a bishop, priest, or deacon, the sacrament of matrimony is administered by the husband and wife to each other; the priest or deacon is merely the Church's witness who blesses the union created by the exchange of consent. The marriage bond, created only by those who are truly free by God's law to marry, is an irrevocable covenant which binds the spouses to each other for life, and the sacrament of matrimony conveys the special grace necessary to strengthen them for lifelong fidelity and growth in holiness.
Christ dwells with the spouses and gives them the grace to take up their crosses daily, to rise again after they fall, to forgive one another, to bear each other's burdens, to be subject to one another out of reverence for Christ, and to love one another with supernatural, tender, and fruitful love. Please read the Catechism, sections 1601-1666, for a fuller treatment of this great gift of grace from God to His children.