"... when you won't open the cats' christmas presents until they are both sat in front of you watching" - aww, :)
"When you spend enough at the vets treating his cancer to have bought a small car - but consider it money well spent for 8 great months"
Yes, RIP Jazzy, run free at the bridge and agree - I always come out of the vets not resenting a single penny of what I've just spent on one or other of the little darlings even if I know it means no more evening wine and only beans for three weeks for me.
My two are going into a cattery for the first time later next year so today I got to interview and appraise the local establishments.
Think the owners were all glad to see the back of me after my 'job interviewing them' and can now add to the list:
"When you ask more questions about a weeks temporary accomodation for your cats than you've ever asked about one of your own new houses (rented or otherwise) before moving in 
Oh, and ...when you up the budget for said kitty holiday accomodation (having sworn cheap and cheapful is fine when setting off that morning) instantly by several pounds per day when you come to the first drafty, dirty, smelly, dank and dark looking establishment.
Seems you get what you pay for everywhere...