Why electrons occupy degenerate orbitals with parallel spins? heard something of stability due to exchange pairing of electrons. someone kindly clarify this matter.
They needn't. This statement is known as Hunds rule and there are many exceptions known.
The usual argument behind Hunds rule is the following: If the spins are oriented parallel, the orbital part of the wavefunction has to be antisymmetric whence the probability to find two electrons at the same position is 0. This leads to a smaller coulomb repulsion than in a symmetric spatial wavefunction.