This may seem like a totally whacked out answer, but the way I see it is as an interaction between Dream Vines and the Water of the Collective Unconscious.
It sounds ridiculously metaphorical, but it's all explained in my totally free e-book
When we sleep, the Dream Vine climbs between our Belief Trees and shows us how to reach mental equilibrium. It shows us the 'best' way forward (the way forward in our life that will bring us to that place we love called "calm").
When you cry, this is actually the collective wisdom of humanity coursing through your body. It's a great thing! It's getting you on the same page as our entire species' destiny.
Yes, I told you it sounds loony, but I'd be honoured if you'd read my e-book
and see if you agree with me.