Why do atoms form bonds?
Out of necessity.
Nature demands equilibrium. It says 1 must equal 1. It say that if 2 of something is necessary for balance then two will be had. Hydrogen is a great example of this. The hydrogen atom contains a single electron. And yet, the first energy level for hydrogen, the first orbit if you will, wants two electrons. Nature responds by two atoms of hydrogen sharing their single electron with each other to balance that first energy level.