Why Do People Like Throwing Others Into The Pool?

Recently, on my friend's blog, I read how she survived from being thrown into the pool...

Gosh! Still couldn't believe how come people have this crazy idea? I know some people do this in my home country, but I am really surprised how it is actually a big tradition for people here (somebody just confirmed about it). I don't understand. They said it's to celebrate. By making a stupid joke? I don't think so.

About a few months ago, my roommate was graduated from her school, and you know what, her friends (and also my friends) had a farewell party for her, plus a special gift: threw her to the pool, 3 times!! I wasn't there, but as soon as I heard about it, I really got mad. If only I was there, maybe I can try or stop them, of course, with consequence I will fight with them. But I don't care. I will stand up straight, against what I dislike. However I still don't get it, why people could see the fun of it?? It's dangerous (could be even serious: one of my other fellow just got hurt, her friends dropped her accidentally when they carried her to the pool, she went to the hospital for some minor injuries, but still had to stay for several days there). And my roommate got several bruises. So? What's the fun of that? What's the fun of hurting other people? Yeah...yeah...probably they will plea that they don't intend to, that the "hurt" part was just an accident. But after that, they will do it again, and again. So? After all, I still don't get it, I still have this question inside of me: Why do people like to throw others into the pool?