[mjevans]

Not a mechanical Engineer, but I can imagine how.

Both for pieces abutted against each other and for E.G. rivet holes, mechanical interfaces have extremely precise tolerances to support a range of possible stresses. Too wide a tolerance in one area can allow deformation and wiggle that applies unexpected forces on other areas. You should also remember that many aircraft are pressure vessels, since they operate at altitudes where the density of our atmosphere is substantially different.