I know the physics, but the bottom line is that as with all physical principles, we know what is observed but know little about the mechanisms that define those observations. Why does the moon ‘grab’ onto a spacecraft (e.g. the Apollo craft) and ‘pull’ it towards the moon? We know it happens, but we don’t really know why.
Because in the vicinity of gravity space is curved. This manifests to us as (surprise) curved movement in 3D space.
I know precisely what you mean. We know the effect of forces like gravity, but not the cause, other than “mass”. But why does mass result in gravity? General relativity explains it as a counterbalancing force to pressure caused by the Pauli exclusion principal, but I don’t really get it. So I guess that means it’s a quantum mechanical effect that causes a curvature in space-time. A space-time curvature being equivalent to a change in velocity, since velocity is a function of both space and time. The question continues. Why do all electrons have the same charge? Why is the speed of light what it is? What ultimately determines all the other physical constants? No clue.