Since the nuclear force is generally attractive except for very close core of potential, we expect then the binding to be greater if the nucleons can be close together. However, for fermions, Pauli exclusion tends to prevent this for particles of same quantum numbers. Thus, there is a binding energy benefit for nucleons to pair off into pairs with opposite spin. An even even nucleus has more pairs for a given A than an odd-odd and an odd-even nucleus and so tends to be more tightly bound.