Why are peroxides unstable but disulfide bridges considered stable? Why are esters stable but thiolesters are unstable?
Some hard data: bond enthalpies (in $\pu{kJ mol-1}$)

$$\begin{array}{c|c|c|c} \text{Bond} &amp; \text{Enthalpy} &amp;
     \text{Bond} &amp; \text{Enthalpy}  \\ \hline
\ce{C-C} &amp; 350 &amp; \ce{Si-Si} &amp; 226\\
\ce{N-N} &amp; 163 &amp; \ce{P-P} &amp; 201\\
\ce{O-O} &amp; 146 &amp; \ce{S-S} &amp; 226\\
\ce{F-F} &amp; 155 &amp; \ce{Cl-Cl} &amp; 240 \\
\end{array}$$

We observe a decrease in bond energy from carbon to nitrogen and from silicon to phosphorus. It is clear and probably have same reason: appearance of lone electron pair of the atom. There is a repulsion between lone pairs, that is in partially compensated by stronger bonds formed by smaller atoms (the atomic radii falls down to the end of the row). When moving from second to third row, this repulsion is reduced thanks to larger size of atoms of the elements of the third row.

The general decrease in energy from carbon to silicon is thanks to increased size of the orbitals of valence level, making them more diffuse with less efficient overlap and longer bond length.

The stability of esters vs thioesters is ruled by different reasons. Sulfur is not as electronegative as oxygen, but when you consider an anion, sulfur anions have larger radius and are less prone to grabbing first positive charge they find. So sulfur anions are more stable intermediates (kinetically), and thus polar dissociation of thioester bonds is easier.