Okay...it seems to me that people have read wayy too much into last night, here's my take on it. The Rock tore his abdomen during his match at Wrestlemania and was in major pain which is why he looked a little "off". Now as for the little conversation Rock and Cena had, go back and watch the clip...Cena says basically "All i wanted was for you to come back", to which The Rock replies, "The reason why i came back was for this moment right here, right now...thank you."
They then shake hands and as they're hugging, The Rock continuously thanks Cena.
The bottom line is, The Rock came back to put Cena over, to say that his own time IS up and that Cena is the number one guy in the WWE. Why? Because he's worked his ass off for 10 years straight and he fucking deserves it.
P.S. @Rockstar BELIEVE IN THE SHIIIIIELLLDD!!! (Sorry, had to get it in there