Why must quarks and leptons combine in twos or threes?

Quarks and leptons must combine in twos or threes due to the principles of quantum chromodynamics and the Standard Model of particle physics. Quarks combine in groups of three to form baryons (like protons and neutrons) or in pairs to form mesons, adhering to the requirement of color charge conservation. Leptons, on the other hand, exist as individual particles or in pairs with their corresponding neutrinos, but they do not combine to form composite particles like quarks do. This structure ensures the stability of matter and reflects the fundamental symmetries and conservation laws governing particle interactions.