I have experienced the same thing, Can't_Stop_Crying. I'm not totally sure (as I am about most things...) but I think it's because if anything at all is going the slightest bit wrong, that's what you (and I) turn to. Another thing that I've found is that sometimes, happiness (or even just feeling "okay") scares us. It's outside our comfort zone because we've felt awful for so long.
I'm no professional anything, and I don't know much, but that's been my experience. When I sit down after that happens and just sit and think, that's about all I can come up with.
Hope this helps, even if only a little.