Question

With respect to `C-C` bonds, why is a double bond WEAKER than a triple bond?

Answer 1

You mean a `"double bond is weaker than a triple bond......"`

Explanation:

The modern chemical bond.........is CONCEIVED to be a region of high electron density between adjacent atoms such that internuclear repulsion is minimized (due to electrostatic interaction between like charges), and a net attractive force results between the electron cloud, and the positively charged nuclei.

And thus, for single bonds, we say that the electrons are shared between each nuclei:

When we introduce multiple bonding, the bonding pairs of electrons are conceived to lie in planes above and below the `"atom"-"atom"` vector.......

In the representation of the ethylene molecule, there are 4 bonding electrons, and this degree of electron density allows closer approach of the bound carbon atoms. A `C-C` single bond has an approx. bond length of `1.54xx10^-10*m`. A `C=C` double bond has an approx. bond length of `1.35xx10^-10*m`. And a `C-=C` triple bond has an approx. bond length of `1.20xx10^-10*m`:

The degree of bonding interaction is not linear. Also see this answer.