Why are single bonds the longest?

The modern chemical bond is conceived to be a region of high electron density that negates the internuclear repulsion between two positively charged nuclei such that a net attractive force results. This echoes notions that we first learn in introductory chemistry, namely that a covalent bond results from the pairing of 2 electrons (and so creating that region of high electron density), and that an ionic bond results from the transfer of electrons between atoms to give discrete positive and negative ions that are bound together in a electrostatic lattice.

While we conceive that electron density is localized between the nuclei in a single covalent bond, on the other hand, double and treble bonds concentrate electron density, i.e. 2 electrons per bond, in planes above and below the vector of the single bond. The positively charged nuclei are electrostatically attracted to such regions of high electron density, and allows closer approach and stability of the positively charged nuclei. Shorter nucleus-nucleus separations, shorter bonds, result.