This week's parsha, Parshat Emor, contains the prohibition of a kohen marrying a divorcee. (21,7)
"They shall not marry a woman who has been divorced by her husband." The Shach explains the reason why a Kohen may not marry a divorced woman. He says that a Kohen symbolizes unity. He brings Klal Yisrael close to Hashem. Aaron was Ohaiv Shalom V' Rodef Shalom. Therefore, he should not marry a woman that went through a process of Pirud - separation. When a woman gets divorced, not only is there separation on this world; the couple's souls are separated in Shamayim, too.
Rabbi Benjamin Blech recently published an article about the Royal wedding and role models for marriage. The following is an excerpt.
In Jewish tradition, the Kohanim, the priests, were required to live by higher standards in order to serve as role models and teachers. A priest was forbidden to marry a divorcee. Not because divorcees were considered inferior. Indeed divorce itself is permitted by biblical law. But the Torah wanted to discourage unnecessary dissolutions of marriage by making clear that divorce carried with it religious consequences. By not allowing a Kohen to marry a divorcee, that helps ingrain in everyone the ideals of marriage for life.
Read full article: http://www.aish.com/ci/s/The_Royal_Wedding.html
Finally, Shirat Devorah posted a letter from the Lubavitcher Rebbe where the Rebbe was asked about a kohein who was contemplating marriage to a divorcee.
...it is certain that the Creator of the world knows best what is good for His creatures, and since He has so strictly prohibited such a marriage, there can be no doubt that it is harmful.
Read full letter: http://shiratdevorah.blogspot.com/2011/05/forbidden-marriage-of-kohen.html