Assuming the "James Comey" now is the same "James Comey" who met President Trump for a private dinner...most Americans can sense "why" President Trump might have broached the topic of loyalty while chewing a piece of filet mignon. While never inquiring about loyalty...it was obvious Donald J. Trump wished to see what fang and claw behind his back slimed and crawled.
In Comey's book...Jimmy-boy explains "why" he was doing all he could to assure Hillary-the-hag became president...along with what he did during his tenure at the FBI to hurt TEAM-TRUMP.
Although Big Don didn't have the benefit of the "hand-written" copy of that book before his private dinner...somehow...due to his Titan-like sagacity...he suspected that seated across from him was a treacherous...cunning...lion-fox-kind-of-dude* whose ulterior motives too horrific to hide; and never would he repose trust in such a running-dog lackey or ask for loyalty from this latest** version of "Carlo".
*Donald J. Trump suspected Comey fancied himself some kind of Prince...lion-fox...wrapped in one big package...cunning...resourceful...and...always ready to do his master's bidding. Hence...between sip of wine and sauteed bean...Big Don asked Comey "why"...and...Comey almost cud-choked. Here was another Messiah telling him...the new-Judas...go and do what you must do quickly.
**In GODFATHER part 1...Michael arrives and tells Carlo he must pay for Santini.