system
Given the following post, respond as if you're commenting on Reddit:
human
Why do Protons and Neutrons have almost the exact same mass?

Is it a coincidence, or is it something fundamental about the laws of physics that makes this so? 

Would the universe look/be any different if those masses weren't similar?
gpt
It is a combination of an apparent coincidence and a something fundamental that we understand.

The proton and neutron are each made of three quarks that are held together by a force called the strong force.  The resulting particle has a mass that gets its contribution primarily from two places: the intrinsic mass of those quarks, and the energy of strong force fields that hold it all together.

The strong force effects produce a mass that is essentially independent of the kinds of quarks involved, so this mass contribution will be the same for the proton and neutron.

For the proton (2 up quarks and 1 down quark) and the neutron (2 down quarks and 1 up quark), it turns out that the intrinsic masses of each of the constituent quarks is just a tiny fraction of the mass contribution due to the strong force effects, so even when added together provide no more than about a 1% modification to the total mass.

So the reason that the proton and neutron have about the same mass is that the up and down quark intrinsic masses are really small compared to the natural scale of strong force effects.  As to why these masses are so small, we don't know.

In fact, there are other quarks that are much heavier, and if you take an object that is like a proton or neutron, but replace one of the quarks by a heavy quark like a charmed quark, you will get an object that is much more massive.  For example, a charmed lambda baryon with an up quark, a down quark, and a charm quark has a mass about 2.4 times as big as a proton or neutron mass.  This is because the intrinsic mass of the charm quark is all by itself about 1.4 times as big as the strong force contributions to the mass.

There are some other small effects, such as electromagnetic corrections to the mass.  These, too, are small, since the electromagnetic force is so much weaker than than the strong force.