system
Given the following post, respond as if you're commenting on Reddit:
human
Why do electrons fill up orbitals when atoms bond? Why are atoms more unstable without full orbitals?

I would like to know what actually causes electrons to act in this manner. I know all about how an atom 'prefers' to have a balanced amount of electrons orbiting it. But why do the electrons 'oblige' and become shared? It seems strange and I can't come up with an answer, nor can my chemistry teacher. I'm not really sure if this is worded correctly, but I am extremely curious, and would rephrase as best I can if the question isn't understandable.
gpt
Short answer:  
Quantum Mechanics  

Long answer:  
Electrons don't have a an exact position as it were, as you might well probably know. Instead they have a wavefunction. These wavefunctions, due to electrons being fermions, must be anti-symmetric and when the wavefunction of two electrons overlap (like they do in atoms) the total wavefunction must describe the positions of both (they must be indistinguishable). This is harder than one might think, considering you suddenly have an n-body problem on your hand with the introduction of 2 electrons and 1 nucleus or 1 electron and 2 nuclei (stable He and and H2+). When considering H2+, we often keep the two nuclei at a given distance and then derive the waveform from there with the two potential wells. With the stable He, we consider one potential well, the individual electron wavefunctions must be anti-symmetric and the total waveform should be symmetric (two fermions can effectively become one boson) and different terms from that arise (using perturbative theory). One such term is what is called the "exchange integral". This generates an attractive force by (more or less) exchanging the positions of the electrons.  

**tl;dr** The exchange integral gives an attractive boost between pairs of electrons.