René Girard is reasonably well known for his idea of mimetic desire, which states that we all want things because we have learned to want them from someone else. The traditional example for this is romantic or erotic love: A loves B because C loves B. This does not only produce particular relations between A and B; it also produces particular relations between A and C. That is, I have unique relationships with people whose desires I imitate, and with people who imitate my desires.
In Theatre of Envy, on the operation of mimetic desire in Shakespeare's plays (Girard claims that Shakespeare is exceptional because he's one of the few playwrights to understand that desire is mimetic and is therefore one of the few to portray it realistically), Girard spends the introduction and the first chapter discussing this dynamic: "Individuals who desire the same thing are united by something so powerful that, as long as they can share whatever they desire, they remain the best of friends; as soon as they cannot, they become the worst of enemies." Thus mimetic desire becomes mimetic rivalry. But it is not only that we inevitably wind up desiring the same things as our friends, or are friends with those whose desires we imitate: we are active agents in sharing desires. We want our friends to desire the same things that we do. Speaking of the eponymous friends in The Two Gentlemen of Verona, Proteus and Valentine, Girard writes: "Whenever they do not see eye to eye, our two friends feel that something is wrong; each one tries to persuade the other that he should reorient his desire in such a way as to make it coincide with his own once again. Friendship is the perpetual coincidence of two desires." The consequence of this effect is as inevitable as the effect itself. On top of friendship's first imperative (Girard sums it up as "imitate me") arises a second imperative, "do not imitate me." He calls this the "double bind" of friendship.
The idea of mimetic desire (and mimetic rivalry) is one we vigourously want to reject, says Girard. As a result, it is masked by assorted claims and ideas and discourses. It very rarely appears in our culture, and when it does it is always disguised. But it cannot be so easily gotten rid of. Short of embracing our mimetic desires and rivalries, we have two options: "total renunciation" of desire, or the production of some kind of monster or perversion. These monsters or perversion have the characteristic of producing new mimetic rivalries later on, or simply recycling the old one in some disguised way.
Girard of course goes on. He talks about learning to desire oneself from a besotted courter (Pheobe and Silvius from As You Like It is his example, and I will parenthetically add that his chapter is the best explanation for the title of that play that I've ever read); he talks about the role of the pander; he talks about alliances forged out of conflicts when the desired object is lost to both parties. But I will rest with this description of his idea for now.