Question

why do half filled or full filled orbitals are very stable

Answer 1

The half filled orbital is stable because it has the minimum of energy ( according to Hund's rule the multiplicity must be at maximum and that is when the total spin is at maximum). If the shell is half filled all electron spins are equal and the total spin is maximum.

The shell with full filled orbital is stable because it also has a minimum of energy.& Next free orbital has more energy