First of all, it's usually before we start our periods, that -some- of us get rather...sad or angry, either of the two. PMS is not something that all of us have, it only affects a few girls. This isn't because we want to act like this, but because of our hormones that mess with our mood. It's like being depressed and having a chemical imbalance, but there's sometimes nothing you can do about it if you don't have any particular treatment.