The joke isn't funny when it's on you...or is it?

Well, yesterday someone I worked with wanted to play a joke on me. He had someone send me an e-mail saying I was going to get fired in the next four weeks. He thought this would be hilarious to see my reaction. So, while I wouldn't have really found this hilarious, the guy sending me the e-mail clued me in on it. I responded to the e-mail by saying they should replace him (the guy that originated this prank) with a monkey because he could do the same job, and that I already had another job lined up in a couple of weeks. The prankster was livid. He sent e-mails to our boss saying I should be fired, and I had a bad attitude and I was on his shit list. He was going off. Before he left I went over and told him I was in on it the whole time. While he calmed down, he didn't think it was the least bit funny. Then he said "Well, I knew about it the whole time. I just went along." Sure you did. We all believe you. In short, while me and my friend got a great laugh out of it, he didn't find it amusing in the least. It's amazing how pranks like that aren't funny when they come back on you. Why is it that some people don't find that funny? If you're going to initiate something, wouldn't you prepare for one to come back on you? Just my thoughts, but I haven't pulled a prank like that on someone since elementary school.