2H2 + O2 + 2H2O2H-H + O=O = 2H-O-H..not shaped like this !!The bonds in the reactants break and the bonds in the products form.The bonds in the H2 and O2 break and the bonds in the water form.
Sulfur and oxygen form a covalent bond. You would expect this, since they are both non-metals. To get an ionic bond you need a metal bonding with a non-metal.
In order to fill their valence shell and become more stable. This question is potentially very deep if you want to get into quantum physics and the general direction of our universe and things like...