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This is an interesting question. I am not sure it's necessary to assume that Romeo and Juliet have suicidal impulses. Rather, their youthful passion takes them to great heights which they sense cannot be maintained. The intensity of new love, especially young love experienced for the first time, is all-consuming, yet it cannot last forever. With time, romance matures into something deeper and richer, yet undeniably less rapturous.
The characters of Romeo and Juliet are short-sighted, as are all young people, about the future. Nothing matters to them but their present passion, which they know is ephemeral. Rather than "settle" for the pleasures of mature love, they choose death.
A similar theme is expressed in the Robert Frost poem "Nothing Gold Can Stay":
Nature's first green is gold
Her hardest hue to hold.
Her early leaf's a flower;
But only so an hour.
Then leaf subsides to leaf.
So Eden sank to grief,
So dawn goes down to day.
Nothing gold can stay.