For example, why is the strong nuclear force strong enough to hole protons and neutrons together.
the nature of forces depend on the available physical systems identified by human kind in the environment and surrounding- if one has various elements on earth/universe having the structure investigated so far -to sustain the structure one has identified the nature, its strength and range of forces of interaction.
To substantiate the above argument -when we investigate the structure of Deuteron nucleus we get the idea of the the force which must operate between the neutron and proton to get a bound nucleus of deuteron- so if somebody asks why it is strong?
one can answer that it must be so strong to keep the structure intact and not fall apart.
The above is a limited picture -the interactive forces must have other properties such that all experimental findings on Deuteron may be reproduced.
e.g. if the deuteron nuclei show a quadrupole moment however small in value the n-p force must have features to theoretically support it.
this is the process of investigation of physical events/phenomena being observed in nature and devising ways and means to define the nature of various kind of forces.
Physics really isn't about that kind of "why" question. We observe what nature does and physics is the framework in which we describe it to the best of our abilities.
Nobody knows. Some people assume the Anthropic principle holds, though I feel this is logically suspect.
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