Sometimes sadness comes on you at the oddest of moments…it isn't a predictable as an eclipse is, but how predictable is an eclipse without the tables and the maths?
Deep sadness is standing in the line of totality, it is complete blackness. you feel empty and you feel alone.
I guess one could extend the metaphor and use partial eclipses to have those moments of sadness that seem to cloud without fully enclosing us. But, that cheapens, lessens and doesn't really cover the loss of all light and life that you feel when the deep sadness comes over you.
the only consolation is that they are often not that frequent.