Romeo does not marry Juliet…
Eventually, (after mourning for far too long) she winds up with the boy she could spend the rest of her life with. And she never regrets it (because that would be foolish).
She marries him, and there is nothing wrong with that (no matter what might have been, if things had worked out differently). And it works, and they comfort each other, and are devoted to each other, and are comfortable (she never wanted to be comfortable, she wanted to be happy). Her life moves on, like a quiet brook, normalcy and peace ("I want to be a star. And not just any star. A supernova, one that goes along and then suddenly it's all fire and power and then that's that" and he was, and after the supernova there is nothing but the black hole). They argue rarely, and when they do it is all silence on her part and withheld words on his (not like it could have been, intense and fiery, all action and insults).
He is all ice, cold blue eyes and pale hair and polite mannerisms and when they fight he holds his words inside until they melt and are gone (nothing like the wild intensity she once almost loved). But she has seen him melt, as rarely as she melts, both raised without the idea of love, both accustomed to neglect, and (sometimes) she knows (thinks) that this is the best thing that could have happened to her, that her father and his parents were right, and they are perfect for each other. Because it would have been foolish (but maybe, oh so wonderful) for her to have ignored the voices of reason in her life, and to have trusted someone who would have ruined (perfected) her future.
…and it is not a tragedy because no one dies.