The Sacrament of Marriage is a covenantal
Union in the image of the covenants between
God and his people with Abraham and Moses
at Mr. Sinai.  This divine covenant can never
be broken.  In this way, marriage is a union
that bonds spouses together during their entire
lifetime.

     The Sacrament of Matrimony signifies the Union of
     Christ and the Church.  It gives spouses the grace to
     love each other with the love with which Christ has
     loved his Church; the grace of the sacrament thus
     perfects the human love of the spouses, strengthens
     their indissoluble unity, and sanctifies them on the
     way to eternal life. (CCC 1661)

The love in a married relationship is exemplified in the total gift of oneself to another.  
It's this self-giving and self-sacrificing love that we see in our other model of marriage, the relationship between Christ and the Church.

     Marriage is based on the will of the contracting parties to give themselves to each other, mutually        and definitively, in order to live a covenant of faithful and fruitful love. (CCC 1662)

Marriage Reflects the Holy Trinity.  We believe that God exists in eternal communion.  
Together, Father, Son and Holy Spirit are united in one being with no beginning and no end.  
Human beings, likewise, were created by God in his own image for the purpose of communion with another human being.