This is related to the fairly strong attractions between nucleons (protons and neutrons), through a force that is called, precisely, the "strong force". In other words, before the nuclear fusion the...
Nuclear fusion releases nuclear energy.More percisely, it releases the vast amount of energy stored in the strong interaction gluons (which hold atoms together).
It depends. For nuclei lighter than nickel, fusion usually releases energy while fission requires energy. For nuclei heavier than nickel, fission usually releases energy while fusion requires energy.