According to Double Down: Game Change 2012, Mitt Romney crossed Chris Christie off his list of potential running mates for a number of reasons that may spell trouble for Christie in 2016. The Times reports:
According to a memo on Mr. Christie from the vetting team, it had unanswered questions on a defamation lawsuit against the governor from earlier in his political career, on a Securities and Exchange Commission settlement involving Mr. Christie’s brother, on names and documentation of his household help, on information from his time as a securities industry lobbyist, and on his medical history.
As for that "medical history," Romney seems to have had a particular issue getting past Christie's obesity, and, according to the Washington Post, gawked at Christie as if he had never seen a fat person before:
“Romney marveled at Christie’s girth, his difficulties in making his way down the narrow aisle of the campaign bus,” the authors write. “Watching a video of Christie without his suit jacket on, Romney cackled to his aides, ‘Guys! Look at that!’”
Hard to believe that this is the same man who once viciously bullied a gay kid.