Why do we cry sometimes? Of happiness, anger, sorrow, regrets or was it because of something else...like emptiness?

How many of us had felt the sudden urge to cry at times? When we don't understand exactly why, but the tears began to cloud our eyes and it trailed down our cheeks too fast for us to hold it back...

I guess it is an indication to hint us that our 'bottle' is full and it is time to convert some 'content' into flowing tears so there is space to store new ones...