Originally Posted by Alden Heathcliffe
I mean, the lame answer would be that they met training and they could say Rollins and Ambrose crossed paths in developmental
Actually, Ambrose and Rollins teaming up already makes sense. Dean Ambrose announced in his debut promo in FCW that the only reason he came there was to see who was the better man between the two of them. They had a hellacious feud, coming to a number of drawn matches and climaxing in FCW's first ever 30 minute match - it had an entire show devoted to it.
After that, Seth went on to become the first ever NXT champion, and Ambrose was, well, Ambrose still angsty about not being called up, etc etc. They were already the two most ambitious and upward moving stars in developmental, who knew each other intimately well and already had huge respect for each other.
Two greatest enemies teaming up to take WWE by storm is such a believable (and awesome) storyline, and it is all there, in the subtext. We can only guess that Reigns was brought in by a combination of Ambrose and Rollins (probably mainly Ambrose), won over by his oratry.
There is also an argument to say that The Shield was CM Punk's making. He cut a promo with Rollins when he was NXT champ, telling him he needed that killing instinct/cutting edge to break the glass ceiling (this was early in Punk's heel championship run). He also gave a similar, but unbroadcast promo at a FCW house show after his match with Ambrose there (this time as a face champ), praising Ambrose and calling him the future. Punk seems to be the guy who put the fire up these two (and, outside of kayfabe, had a BIG hand in them getting hired), which partly explains why they have never felt the need to attack him.