What's wrong with people?

Yesterday night I went out of Rober's place after having dinner there and pretty tired because it's been a rough week with a lot of studying involved and few hours of sleep. So I'm really looking forward to getting to my house and hit the sack (as Anneke would say). I go to where my bike is parked and unlock it. But surprise!! I can't ride it because I have a flat tyre. So I take a look at it and realize it doesn't have a hole or anything but that someone had left the air out of it. Conclusion, I have to walk all the way back to my place carrying the bike with me but unable to ride it.And now I ask myself, why would someone want to deflate it? I mean, what's the point in doing so? well, I guess I won't find an answer for that because only the person who did it knows.