Why does the sun, despite having a similar composition to Jupiter, shine as a star while Jupiter does not?

Well, darling, the sun shines as a star because it's a hot mess of nuclear fusion going on in there, causing it to emit light and heat. Jupiter, on the other hand, is just sitting pretty, not feeling the need to show off its glow-in-the-dark qualities like the sun. Trust me, dear, if you had that kind of energy burning inside you, you'd be shining bright like a diamond too.