I think it feels serendipitous only if you think of "sweet" taste (or for that matter any taste) as occurring in this giant continuum of tastes where hitting a

I think it feels serendipitous only if you think of "sweet" taste (or for that matter any taste) as occurring in this giant continuum of tastes where hitting a specific spot on that continuum is probabilistically impossible. Ultimately, we experience tastes thanks to chemical receptors on the tongue, and as long as a substance triggers those receptors we experience a certain taste. Now of course if triggering needs a very very specific combination of atoms, then it is not very probable, but we know especially from drug research that you can use similarities in molecular structure or sometimes in say the electron cloud of the molecule to perform this triggering. Caffeine for eg is a selective adenosine antagonist that fools the body and binds to adenosine receptors. Now of course int the case of caffeine the structures are somewhat similar, but you could imagine similar stuff happening with other molecules too If you think of it as that sort of problem, it's not that surprising that many different types of molecules might achieve the same effect.