So, you've probably noticed that characters like Troilus, Cressida, and Pandarus are always running around talking about themselves as if they're familiar with their own myths, even though they obviously don't know that they're literary characters. In other words, Troilus and Cressida are constantly making references to their doomed relationship, even though they have no idea that it's doomed.
It turns out this is a pretty big deal because it's one of Shakespeare's most famous and heartbreaking uses of dramatic irony. Dramatic irony happens when we, the audience, know way more than the characters do, so that the characters' words and actions have a different meaning for us than they do for the characters onstage.
You want a specific example, right? Here you go. (And by the way, this is just one of many, many examples of how Troilus and Cressida promise to be faithful forever.)
If ever you prove false one to another, since I have
taken such pains to bring you together, let all
pitiful goers-between be called to the world's end
after my name; call them all Pandars; let all
constant men be Troiluses, all false women Cressids,
and all brokers-between Pandars! Say, amen.
We don't want to spoil the ending for you, Shmooperinos, but this is exactly what's going to happen in the play. We know this because Shakespeare didn't invent the story of Troilus and Cressida. It was passed down to him from medieval literature, especially Chaucer's famous poem Troilus and Criseyde. This love story was so legendary in Shakespeare's time that "Cressida" was already a name associated with unfaithful women, "Troilus" with fidelity and loyalty, and "Pandarus" with, uh, pimps.
No wonder Shakespeare scholar Anne Barton says this moment is "positively hideous in its irony" (source). In this case, Troilus and Cressida really do believe all those promises they make to each other and, as we watch them struggle to stay together, we know that they are completely powerless to avoid their fates, no matter how hard they try. Cressida will betray Troilus because her story has already been written. And there's absolutely nothing she can do to change that.
To be honest, we think it's kind of gut wrenching to watch.