Why is people falling over so funny!

I really cant help laughing at people falling over, i really want to stop and help them but i am trying so hard not to laugh in there face that i cant, Luckly the last few times it has happened it was in a densley populated area and lots of people ran over, im sure if they were really hurt i would kick into life saver mode but for now i will keep giggling and hope they dont notice.