system
Given the following post, respond as if you're commenting on Reddit:
human
Why do left-handed fermions transform as doublets and right-handed particles as singlets?

I don't know how generally applicable this is, but it is at least true in electroweak theory. The answer *seems* to be "it just is this way" but if so, what is the practical upshot of the different forms of the left- and right-handed particles?

edit: in the title I say "fermions" first and then "particles". I mean fermions in both cases.
gpt
This is true for all the standard model fermions, but there isn't really a known reason for it.

What you're asking kind of gets at the foundation of the standard model, namely the fact that it is based on gauge invariance under the group U(1)×SU(2)×SU(3), where the SU(2) part of the transformation applies only to left-handed fermions. In principle, we could have had a universe based on U(1)×SU(2)×SU(2)×SU(3), i.e. with an extra SU(2) group that applies to right-handed fermions as well, but we don't. The standard model doesn't really bother to explain why, it just starts with the particular gauge symmetry we know to work and proceeds from there.

Perhaps some more fundamental theory gives a reason why the gauge symmetries are as they are, but anything of that nature would be speculative right now.