It seems John Cena needed a tag team partner, and he asked The Rock to be that Tag Team partner against R-Truth and The Miz at Survivor Series. The problem was he had to wait for The Rock to give him his answer. I think everyone knew The Rock would accept as he hates The Miz and R-Truth as bad as he does Cena. I guess you have to look at it's the lesser of the two evils.
By having Cena and The Rock team-up it is a great way to lead into WrestleMania which is coming up and the not so distant future. Look for The Rock and John Cena struggle as a tag team, but it should be fun to watch. Below is a video where The Rock tells John Cena he will be his partner.