Nuclear binding energy is the energy required to separate an atomic nucleus into its constituent…

Nuclear binding energy is the energy required to separate an atomic nucleus into its constituent protons and neutrons, or, equivalently, the energy that would be liberated by combining individual protons and neutrons into a single nucleus. For example, the nucleus of the hydrogen isotope deuterium, which is composed of one proton and one neutron, can be separated by supplying $2.23$ million electron volts (MeV) of energy. Conversely, when a slowly moving neutron and proton combine to form a deuterium nucleus, $2.23\text{ MeV}$ energy is liberated in the form of gamma rays. Why is the energy needed to separate an atomic nucleus into its constituents?