Why Cu2+ is more stable than Cu+ ? Please explain in detail.

Stability depends on the hydration energy (enthalpy) of the ions when they bond to the water molecules. The Cu²⁺ ion has a greater charge density than Cu+ ion and thus forms much stronger bonds releasing more energy. The extra energy needed for the second ionization of the copper is more than compensated for by the hydration, so much so that the Cu⁺ ion loses an electron to become Cu²⁺ which can then release this hydration energy.
In aqueous solution Cu⁺ disproportionate to Cu²⁺ and Cu
2Cu⁺------>Cu²⁺ +Cu