This isn't a joke this is true. You don't have to cry half a gallon to dehydrate yourself. I'm hoping someone else can back me up here about dehydration headaches....
When I work out at the gym and do 30 minutes of cardio, I do sweat a bit but not "gallons". Not even enough to form drops of sweat that fall... it's just a glow of perspiration. If I don't drink a full bottle of water during that time, I get a headache within an hour.
Same when I cry. Every time I cry for any decent period of time (more than just a wee tear) I get a headache within an hour if I don't have a big glass of water.
Your body is totally reliant on hydration and a headache is the FIRST sign of dehydration. If you still don't believe me, give it a try, seriously. Next time you're having a big cry, drink several glasses of water during and after, and I'll bet you don't get a headache.