What causes oil and water to be unmixable?

Oil particles are non-polar which means that they aren't attracted to one another. Water molecules are polar, so they always try to find one another. Because water molecules try to stick to one another and because they are more dense, they will sink to the bottom and form a layer of water underneath the oil. The oil molecules don't really care about being together, but because they are less dense, they will float to the top and form a layer of oil on top.