The root causes of reviling I have observed are:
1) Fundamental lack of understanding
It is fascinating to see how badly many people misunderstand even some very basic things about those they oppose. Over and over, I have read statements like the following:
Mormons hate gays.
Anyone who voted for Prop. 8 is a bigot.
The most severe example was in a comment just yesterday, on a blog that generally is not one of the more extreme:
Remember, we are talking about people who literally do not want homosexuals to exist!
On the other hand, I have read things like the following from those who supported Prop. 8:
Satan is behind the opposition to Prop. 8.
All Mormons who didn't contribute to the campaign and/or voted against Prop. 8 (no matter if they said anything publicly) don't accept the prophets and should lose their temple recommends.
Gays are out to recruit my kids to be gay. It's not about them; it's about converting straight kids to become gays.
This is only a tiny sampling, but it is crystal clear to me that these people simply don't understand those that believe differently than they do. They might even know a few of these people (e.g., homosexuals on one hand and Mormons on the other), but they definitely don't understand them as a group - particularly since they tend to assume that the good people they know on the opposite side are the exception to the rule.
2) Personal prejudice
Many people grow up with a particular prejudice being taught in their youth and adolescence. This leads to biases and stereotypes about religious adherents, entire races, political partisans, men and women, socio-economic status, etc. Much of what I have read shows this underlying assumption from the impressionable years of youth - and this is not confined to one side or the other. For example, many Mormons and gays have a well-developed persecution complex, often for solid reasons rooted in actual events of discrimination against each group as a whole. It is especially hard for these people to view something like Prop. 8 with anything even resembling impartiality, as their personal experiences tell them that their opponents have the worst possible motives.
3) Prior Pain
This is similar to #2, but it goes a little deeper in what I have read. Many people who are responding to the result of Prop. 8 - especially those who lost the vote (those who support gay marriage), but also Mormons who now are the object of protests and scorn - have been hurt previously in very real ways by their status as Mormon and/or homosexual. They have experienced true and virulent homophobia and hate. Those prior experiences, like the teachings of their youth, make it very difficult to distinguish the nuances inherent in differing perspectives - and make it much easier to ascribe hidden motives and unexpressed loathing to people who really do bear no ill will personally to those with whom they disagree. These people, particularly, have an excruciatingly difficult time accepting that those who disagree with them still can love them. Their experience teaches otherwise, and experience trumps all else in most situations.
4) Impaired Perspective
Of all the reasons I have seen for the reviling and vitriol I have read, I believe this one is the most fundamental - and the only one that when corrected can lead to the eradication of #1 and the overcoming of #2 and #3. I truly believe that the recognition of every single person as a child of God and, therefore, a brother or sister with identical eternal potential is the only achievement that has the power to eliminate reviling completely and irrevocably. This goes beyond just believing that all are equal in God's eyes - although that is a great place to start. This includes believing ALL can rise above their natural, human weakness - in Christian terms, believing that the Atonement of Jesus Christ is capable of overlooking ANY inadequacy or flaw by focusing solely on the condition of the heart - believing that God sees with unimpaired perspective. Ultimately, it includes a belief that the object of existence is to develop the characteristics of godliness articulated in the Sermon on the Mount and other scriptures and become Christlike, since becoming a new (wo)man in Christ provides the perspective necessary to see divine potential in all others.
Matthew 7:1 is compelling in its simplicity:
Judge not, that ye be not judged.1 Corinthians 13 speaks of this type of charity and forbearance, with verse 12 being an especially powerful summary of the difficulty in understanding others enough to not revile them - particularly a phrase that generally is overlooked as spoken by a prophet:
For now we see through a glass, darkly . . . now I know in partRecognizing that we don't see and understand fully - and developing the humility and charity necessary to cut others some slack, not judge them and accept what they say as honest and sincere (even while disagreeing ) - is critical to avoiding reviling. Similarly necessary is the meekness to not need to "win" every discussion - and not see everything as a battle between us and an "enemy". Sometimes - I would say often - it is fine simply to agree to disagree and move forward in love and mutual respect. In order to do so, however, we must see others as our equals in the eyes of God - and that is not an easy thing to accomplish. It does not come naturally. Rather, it must be a conscious decision and constant effort.
Regardless of the effort required, I promise it is worth it.