"Laugh and the world laughs with you, weep and you weep alone". As the saying goes. Humour does add some tonic to our life. And that's the reason people crave and appreciate humour. People who act as a source of humour are rewarded by society. This of course, encourages us to BE the source of fun. Making jokes about yourself is hard especially if you're not very confident about yourself. Making jokes about science, comics, etc could fall flat with those not in the know. So the easiest way to make jokes is to make fun of others' faults. It doesn't require a lot of courage, it doesn't require a lot of knowledge, It doesn't require any effort on either the joke maker's or the hearer's part. So this is probably the most common kind of joke made. The issue with making fun of others is that others have feelings (unlike a social situation or a science fact or a wordplay). Many people (especially children), don't realize that someone may be getting hurt each time he/she says a cruel joke. But most of us realize it at some point, and stop. Joking among friends is not so bad as its within your circle and feelings are not hurt too much. There are however, adults too who make fun for the sake of putting down a person. They may do this deliberately to belittle the person because of some former vengeance. Or they may do it simply to look better in comparison. At the end of the day, what we say and how we react to things defines who we are. People making fun of others to cause hurt may be suffering some form of inferiority complex. On the other hand, someone who gets hurt by a stray comment could also possibly learn not to be bothered too much by snide comments.