Occasionally you don't have lunch and it's 12:05 on a Friday and you're freaking starving. Not real starving, but you know the American version of that (your stomach started growling 10 minutes ago). That's when it hits you! The smell of those 2 all beef patties with onion, pickles, and special sauce start making their way over to your cubicle. You look out the window and there it is...the luscious Golden Arches of McD's!
You're a hard worker, you have a cubicle with a window Christ's sake, you deserve it! The problem is you're 35, just bought a house and have a newborn at home. Money is tight, but what would one little meal hurt? Just put it on the credit card and deal with it later.
This happens a lot more than once a month, but let's keep it simple and conservative. You slip up once a month and buy a Big Mac meal for $6.71 and put it on your credit card knowing that you'll have to keep the balance since you don't have the money right now. Every month for the rest of your working career (30 years) with a credit card interest rate of 17% will build a balance to $83,484 by the time you retire at 65!
I don't mean to beat up on McDonald's (they have great cheap coffee), but this is an example of so many things in your life that you buy and never give a second thought to. The $4 coffee, the $7 martini, the $6 pack of cigarettes, the $300 Coach purse (I own their stock, keep buying!), the $8 movie, etc. etc. etc.
It's like a death by a thousand paper cuts. They all add up and they all add up to an ass load of debt if you buy this shit with a credit card. Now if you really want to get depressed look at how much that cable bill and interest on your car payment cost you over your whole life. Assume a 7% return on your savings...I'm sorry, I'm all out of rope.