Why do girls cry all the time?

Hormones, their stupid periods, and because of a little thing called estrogen. As a guy you just have to deal with it, because most girls with feelings cry, and sometimes it's best to get one of her girlfriends to help out and just stay out of it.

Stress, MEN, hormones, MEN, chocolate deprived, sad movies, MEN.

That's not fair, all the time is an exaggeration. For one thing though, it's socially acceptable, and even encouraged in some circumstances (even stoic lorli will loosen a tear or two for officer speeding-ticket-giver). It also releases stress very effectively. I assume you're really asking why girls cry in public, or in front of you.

Frankly, you should learn to deal with it. understand why she's crying (bad day, someone mean...) and address that.