Pence will never be a threat to Trump. But Christie and Gingrich? They have strong personalities just like Trump and Trump probably didn't want the competition. Why is he afraid of it?

The top of the ticket never makes a selection that upstages them. You aim for competence, not star power. It's sound strategy. If #2 outshines #1, the world sees that a mistake has been made .In Trump's case, he got neither competence or star power.