Although I'm sure others could offer different alternatives, I would say that Romeo is experiencing a bit of happiness in his despair throughout Act 5, Scene 3 of Romeo and Juliet. Why a bit of happiness? Romeo gives his own explanation in lines 88-90:
How oft when men are at the point of death / Have they been merry! which their keepers call / A lightning before death. O, how may I / Call this a lightning? (5.3.88-90)
In other words, even in Romeo's despair, he feels "merry" because he is with his love. At least they will end their lives together and that, in itself, is a blessing. Even further, Romeo actually jokes with the dead Juliet, further showing his love for her:
Ah, dear Juiet, / Why art thou yet so fair? Shall I believe / That unsubstantial Death is amorous, / And that the lean abhorred monster keeps / Thee here in dark to be his paramour? (5.3.101-105)
In other words, Juliet still looks so beautiful that Romeo is asking of the monster of Death is keeping Juliet there to be its own lover. Only the reader knows that Juliet is, in fact, not dead.
However, anyone about to take his own life or anyone simply in the presence of a dead lover is going to be in a state of despair, and Romeo is no exception. He states, "Here, here will I remain / With worms that are thy chambermaids" (5.3.108-9). What a gruesome image! It isn't long before Romeo is speaking of "everlasting rest" and "world-wearied flesh," two common images of the despairing heart. Romeo even uses his lips to "seal with a righteous kiss / a dateless bargain to engrossing death!" (5.3.114-115). Therefore, despite all of the honor and joy Romeo might feel in finding final rest near his love, the fact remains that Romeo and Juliet can never again experience their love among the living.