What causes nuclear fusion?
Nuclear fusion is the joining of two atomic nuclei, to make a new, larger nucleus. In order for it to be able to do so, high amounts of pressure and thermal energy (heat) need to overcome the force of repulsion between the two protons in the nucleus, since like charges repel.
Nuclear fusion can create energy because the large, combined atom is somewhat lighter than the individual small atoms put together. This difference in weight produces energy, as E=MC squared states (mass times the speed of light squared equals energy). This is the source of energy of large stars including our sun.