Why can some ions have different valence?
An empty, half-filled or fully-filled electron shell is especially stable.  $\ce{Fe}$ metal is [Ar] 3d6 4s2.  $\ce{Fe^{2+}}$ emptiess the 4s-shell.  $\ce{Fe^{3+}}$ give that plus a half-filled 3d-shell.  Vanadium II, III, IV, V.  Now try chloride, hypochlorite, chlorite, chlorate, and perchlorate.

The most stable oxidation state depends on the medium and added ligands, if any.  Cu(I) is a Lewis soft acid. It is stabilized by soft bases like pyridine, $\ce{MeCN}$, olefins, $\ce{I^{-}}$.  $\ce{Cu^{2+}}$ is a hard Lewis acid.  It is stabilized by hard bases like ammonia, water, $\ce{Cl^{-}}$.  Dissolve $\ce{CuCl2}$ in pyridine or $\ce{MeCN}$ and you have a powerful oxidizing agent. Dissolve a $\ce{Cu^{1+}}$ salt in water and it disproportionates into $\ce{Cu}$ metal and $\ce{Cu^{2+}}$.  Put $\ce{Cu}$ metal and $\ce{Cu^{2+}}$ into $\ce{MeCN}$ and you get $\ce{Cu^{1+}}$.