Well, they perceive that there whole point of alive has vanished. Romeo tells the apothecary that he is "weary of life" and in Act 5 sc 3 Juliet , seeing dead Romeo beside her, instantly wants to be dead too, "Poison, I see, hath been his timeless end:O churl! drunk all, and left no friendly drop To help me after?" I think the only struggle here is that these two realise they can never be together. Their union was the be all and end all of their existence. If they can't have it in life, perhaps they can have it in death.