Friar hopes that the marriage can bypass that hate between both sides of the family he believes that peace and love between the couple can help the other people of the family.
The subject of the poem is life. When you look at it in depth, its entirety is a metaphor for the passing of life. Nature's first green is gold (the birth of a child, or new life), her hardest hue to hold (innocence passes fast with life, no matter how hard we try to hold on to it). Her early leaf's a flower; but only so an hour (again with the quick passing of time for life.) The leaf subsides to leaf. So Eden sank to grief (death at the end of someone's life and the mourning that comes with it, if only a second to the hour of life), so dawn goes down to day (mourning is over, and the days continue after that someone passes and everyone has mourned). Nothing gold can stay (life is valuable, like gold, and vanishes much in the same way).
