JM- no offence matey but surely after going through all of your ATPL's, the flying, the IR, the HUGE debt and blood,sweat and tears that have got you to a position for interview..you could come up with a reason why you want to fly for an airline..
My advice is to think back to the times when u were going through hell, when u asked yurself, why the hell am i doing this..and there must have been a reason for continuing.
Include stuff like 'passion for flying' etc but as pointed out dont be false, they can see right through!
GOOD LUCK!!