It is said that since the energy of a bound system is lower
than that of an unbound one, its mass is lower as well.
This being the explanation why when a proton and neutron
merge, it has to release energy in the form of nuclear
binding energy. Can anyone tell me what concept in physics
does this belong to? I mean.. When two people copulate,
they are binded. But they don't lose any mass and
converting to energy. Or when you stick two bubblegum
or any objects together, you don't get mass lost and
conversion to energy. So why do the nuclues do that.
Is it directly connected to the strong force? What is
its relationship to the strong force? What other examples
where bind energy occurs without the strong force. I want
to understand "why" the energy of a bound system is
lower than that of unbound one and the mass is lower
as well.

Thanks.

David