This play, like so many other plays, ostensibly deals with the central theme of how we can achieve our own personal desires and goals against a bigger and larger backdrop of the interests of the state and society at large. This of course is something that sounds the death knell of the relationship between Troilus and Cressida, as they are parted by forces bigger than themselves. However, what is interesting abotu this play is the way that this is not simply tragic, but that Shakespeare seems to go out of his way to use bathos or anti-climax to present a bleakly depressing view of life and of love, showing how the love of the two title characters is actually based on nothing more than lust. This of course gives rise to one of the most famous quotes from this play, which comes from Thersites, who says, "all the argument is a whore and a cuckold."
One of the major devices therefore that Shakespeare uses to present love is anti-climax. For example, consider Act V and the final battle, in which we expect Troilus to be avenged for the loss of his beloved and Hector to have a duel with Achilles. The way in which Achilles instead ambushes Hector when he is unarmed and kills him and Troilus is not avenged seems to deliberately present a gap between our expectations and thoughts of love and how it is and should be presented and the much more depressing and bleak reality that Shakespeare seems to be pointing towards. Love, in this play at least, is not a force that leads you to kill yourself to be with the other, as in a play like Romeo and Juliet. It is instead presented as a fickle emotion that is used and abused.