God almighty, some of the stupidity in this thread is just mind boggling. Pull up a chair kids, time for a lesson.
Let's start with clean loss, which is something a lot of people are getting confused about. A clean loss means a wrestler loses in a fashion that is devoid of interference, outside circumstances, distraction, anything. Its just a simple no nonsense pin in the middle of the ring. Goldberg beating Hogan? NOT clean. Curt Henning came down to ringside, followed closely by Karl Malone and DDP. Malone gave Henning a Diamond Cutter while Page taunted Hogan. Hogan, who had just hit Goldberg with a leg drop, was distracted and was pointing and shouting at Page, drawing attention away from his opponent, who when recovered, hit Hogan with a Spear and Jackhammer to win. Nothing about that was clean.
Second, we move on to everyone's favorite golden boy. Cena has, in his time, put over SOME opposition cleanly. HBK, Trips, Batista, etc. However, what Cena is absolutely terrible at is SELLING the loss. That is a big thing people in this thread don't seem to get. You can lose in an unclean fashion and still put your opponent over huge (which is what happened with Edge, for example, or Punk). The babyface absolutely has to make it seem to the viewers, in attendance and at home, that their loss means something, and at the same time that the heel's win means something. The heel won and the babyface lost, so both parties must do their part to sell, to the audience, the ramifications of what the outcome means. For the heel, a win over the babyface, clean or not, means that they got the better of the babyface and that the villain is in a position of strength, and the babyface must sell that the loss shows them in a position of weakness. This is absolutely CRUCIAL. It puts the situation in danger of becoming won out by the side of evil, and therefore gives good a reason to overcome it. Overcoming the obstacle is what provides us with adversity, from adversity we derive drama, and thus entertainment.
Cena's problem, with almost all of his feuds as of late, is that he doesn't sell the loss. The villain beats Cena, then Cena, typical to his character, goes out and says "ah well, better luck next time for me. So what if I lost, I owned up to it. I rise above hate, and I know in my heart that I'm the best in this industry." Now the victory the heel gained means nothing. Why? Because the babyface he beat has flat out stated that the win meant nothing, the loss meant nothing and there's no reason for any sort of feud to continue. THAT is a problem. Effectively, Cena is stealing the heat from his opponent and giving it back to himself. When the heel has no heat, where's the interest in seeing them get beaten? They're just another guy. Its like you have to start all over again. Where's the build up of the heel as a legitimate threat or contender who the fans are supposed to take seriously? There isn't any once Cena gets on the mic.
He constantly does this. Nexus, Punk, Brock, now Ziggler, all of them. Either he wins and celebrates it as a big deal, or he loses, and sluffs it off like it wasn't a big deal. This is a big problem with the Cena character. He's designed to rise above hate, in other words, to not make it seem like losing gets to him. Its the character's way of being a good role model to children-you know, don't let opposition get you down, fight to achieve your goals, etc. Unfortunately, while it might be a good message to kids, it makes the character so incredibly boring. His saying that a crushing loss won't affect him actually becomes a virus that ends up fucking over the guys he WORKS with as well. Its not just that the Cena character is dying, it actually kills the guys he goes up against as well.
Remember Punk in the summer? "Hey Cena, why should you get a title shot, I've beaten you 3 times. I've always come out on top against you." Alright, Punk is trying to build up his heel character by showing that the babyface has constantly lost to him. He's establishing that the babyface has a real obstacle to overcome, which would be actually beating him. Then Cena gets on the mic. "Punk your whole 300 day reign has been irrelevant, I don't care if you give me another title shot, I'll just get one later anyway." Wow. Punk has beaten Cena 3 times, including once as a heel, so Cena counters that Punk's reign has made the title worthless and Cena doesn't care that Punk has beaten him. Way to make the heel seem credible. I really want to see him get beaten now that you've told me his title means nothing and that even though you've lost to him 3 times, that means nothing as well. In fact, he even says that he'll end up with another title shot later anyway, which means that regardless of the outcome of the match, Cena will ultimately come out on top. So Punk now has a worthless title, worthless wins over the top guy in the company and Cena will get what he wants eventually anyhow. All of these things meant something, until Cena said they didn't. He failed to play up the importance, and he's the top guy. If he thinks they're important, then his fans and the audience will think they're important as well. He doesn't, so that kills it. Why would you want to see a Punk/Cena match, as a casual fan, when Cena has already said that he doesn't care that he wins or loses, and says that his losses to this guy from before didn't mean anything? That means that if he loses now, it won't mean anything, in which case why the fuck are they wrestling?
For the audience to be interested in a feud, they have to see the heel look like they're are in an advantageous position that the babyface needs to overcome. Part of the heel's advantageous position is the babyface acting like they're a real threat. When the babyface doesn't, the heel is no longer in an advantageous position and the interest in the match just dies. All because the babyface didn't act like the threat was credible or legitimate. And its in this area that Cena is a grievous offender. Forget putting people over cleanly, he needs to fucking act like what he's involved in has a point.
TL;DR version-Cena loses enough, he needs to stop no selling his losses.