For those who play string instruments, why do harmonics only sound clear on select parts of the string?

If you lightly put your finger down on a random part of the string, the sound will come out rusty and mangly. Why is it that only a few places on each string have that ';right'; harmonic sound?For those who play string instruments, why do harmonics only sound clear on select parts of the string?
From playing the violin I would say a combination of the tension of the string, the ridges on the string against the bow.For those who play string instruments, why do harmonics only sound clear on select parts of the string?
that ';sweet spot'; can actaully be different with different guitars. i hit the string with my thumb and just the right amount of bend to do this