What is the link between the theme of emnity and Friar Laurence whose role shapes the end of "Romeo and Juliet"?

As a play of contrasts, Friar Laurence fits perfectly into this motif of "Romeo and Juliet."  In his soliloquy of Act Two, Scene 3, the friar reflects, "For naught so vile that on the earth doth live,/But to the earth some special good doth give..."

Later in this same soliloquy, he states,"...vice sometimes by action dignified."  So, by marrying Romeo and Juliet against his initial negative reaction to Romeo's impetuous request, Friar Laurence hopes that by joining the young lovers of the feuding families he can effect reconciliation: "For this alliance may so happy prove,/To turn your households' rancor to pure love."

Ironically, however, Fate foils the plan of Friar Laurence by causing Romeo to not receive the priest's message which states that Juliet is not really dead as she lies in the family's catacombs.  This twist of Fate, sadly, does unite the families.  But, instead of uniting them in amelioration, it unites them in grief.