Question

What theoretical explanations exist for the fact that there are three generations of leptons and quarks?

I'm not so much asking why there are exactly 3 generations, but rather what makes electron, muon and tau differ. Also, since the three families of quarks don't have to be a priori related to the three families of leptons, I'm interested in answers for either quarks, leptons, or both.

Answer 1

here is another argument (disclaimer: I'm an experimentalist):

e.g. Wikipedia states that:

"Direct" CP violation is allowed in the Standard Model if a complex phase appears in the CKM matrix describing quark mixing, or the PMNS matrix describing neutrino mixing. In such a scheme, a necessary condition for the appearance of the complex phase, and thus for CP violation, is the presence of at least three generations of quarks.

the same article also says further down:

The universe is made chiefly of matter, rather than consisting of equal parts of matter and antimatter as might be expected. It can be demonstrated that, to create an imbalance in matter and antimatter from an initial condition of balance, the Sakharov conditions must be satisfied, one of which is the existence of CP violation during the extreme conditions of the first seconds after the Big Bang. Explanations which do not involve CP violation are less plausible, since they rely on the assumption that the matter