In researching the topic of nuclear fusion, I have a few doubts related to the mass defect, 'negative' potential energy and resultant energy released as a by-product of nuclear fusion.
When two hydrogen nuclei (for instance) fuse to form a helium nucleus, the nucleons experience extra nuclear energy which in turn increases the binding energy between the nucleons. What is the origin of this extra energy? What is the cause of this energy? What accounts for this extra energy?
I have heard that during the fusion, the hydrogen nuclei have negative potential energy which in turn causes a mass defect in the product nuclei. How is negative potential energy possible? I have also read that it is the differential between two states of mass. And if so, what exactly causes the 'negative' potential energy in the hydrogen nucleus. Is it due to the extra nuclear energy experienced by the nucleons of the helium nucleus and that they reduce their mass or something else? Why is energy released in fusion according to $m =E/c^2$ ?
Please answer a lot in detail with an example of a fusion specimen as it is difficult for me to understand just in mathematical terms....