system
Given the following post, respond as if you're commenting on Reddit:
human
Why is stable matter only composed of protons, neutrons, and electrons? Why aren't there other stable combinations of elementary particles?
gpt
Those have the lowest energy for given decay path (meaning: although a proton has higher energy as an electron, there is no path leading to its decay to an electron). A free neutron is btw. not stable, but will decay into a proton (and other particles). Neutrons are only stable inside of a nucleus, because in a *bounded* state, it might be favorable for a proton to decay into a neutron (because too many protons in a nucleus make it unstable).

Keep in mind, that a proton is also not a fundamental particle, but consists of 3 quarks (which are fundamental, but only exist bound together).

So the fundamental answer to the question "why is the universe like it is?": because it is the most stable configuration under the given circumstances.