Manish,

That's a deep question for which there is no simple answer (IIRC). The Stern-Gerlach experiment shows that there is a spin-like phenomenon present in fundamental particles - but (naturally) it cannot arise from actual rotational motion of those particles.

Spin appears, like charge, to be a fundamental quality of matter.

Note, it is also misleading to think of electrons 'orbiting' a nucleus - they cannot do so in a common-sensical manner without losing energy (accelerating a charge) and being rapidly destroyed. 

Thank you James, for your attempts to answer the question.

Regarding the note you have mentioned: I guess you are talking about Rutherford's atomic model which is unable to define why atoms are not rapidly destroyed as they must as per common sense. THIS WAS THE REASON why Rutherford's model was discarded and Bohr's atomic model prevailed over it. Bohr's model talks about quantisation, and of course, quantum mechanics can be understood only if we dont apply our common sense.

I am of this view. Please do correct me if I am wrong..