My theory is that it's a relic from the past. I did some research on honor-based cultures and it turns out that the environments where law is not working have evolved their own rules of dealing with offenders. These rules may seem like an overkill for an individual (e.g. you need to challenge to a duel someone who says anything bad about you or your close ones) but they might be beneficial to the society as a whole, in the sense that the offender knows his victim will be forced to retaliate. Of course this system was abused but it looks it was the best our ancestors could come up with.

Similarly, I think many people who pretend to be offended today don't feel that much emotional pain but are trying to introduce a change in behavior of the offending party in line with their expectations. Many times they feel they are not respected or their other emotional needs are not fulfilled. I usually try to deal with it in a frank way: "Look, I like you a lot, and that's why I allow myself to joke with you in a playful way, but it's by no means a lack of respect. If you don't like these jokes, sure, I can readjust our relation accordingly."

People take life too seriously. As if anyone is going to make it out alive!