Try to keep it as simple as possible, as I am still completing school. Just wanted to get an explanation.
There is no solid theory on why the occurrence of even number of either nucleons in a nucleus of an atom is stable. Only through experimental data have we been able to observe this phenomenon and the concept of magic numbers.
Even some approximations and theories such as Semi-empirical mass formula fail to explain this occurrence.
But we cannot generalize this observation for all elements as some odd-odd stable isotopes are also present, for example :
2H1, 6Li3, 10B5, 14N7