I have a couple of suggestions...
Q. Why did you buy that RF-4C, when you already have 15 other marks of Phantom waiting in the loft?
A. DAMAGE LIMITATION - I was going to start collecting P-51s today, and you've seen how many of those there are in the shop that I don't have yet!
Q. Why did you have to buy another kit - we have bills to pay?
A. EXACTLY my sweetness, just think of all the money we're saving in heating bills with all that plastic loft insulation up in the roof!
Q. A 1/72 B-1B - What did you get that for? You don't even collect B-1s!
A. Well, I was considering the Revell 1/48 B-1B, but I thought this would fit on the shelf a bit better.
and the classic...
Q. Why have you bought yet another model aeroplane?
A.BECAUSE I CAN!
Oh, and it's better than spending my money on beer/other women/crack/gambling etc. etc. etc.