Sometimes we just need a good cry. I tend to be sad after having a good happy time, and I often think it might be because when I feel the happiness,it brings to mind all the years I missed out on happiness because of the abuse I grew up with that followed me all my days.So many years wasted..filled with confusion and turmoil.And that makes me sad.
I have moments of sadness often and I can't ever understand why so I think about it, trying to come up with an answer. Above is one theory.

I hope you are able to let the tears come, be cleansed, and feel happiness again.This too shall pass...Faith