Just because something is on sale or you have a coupon to use does not always make it a good deal. I know because I used to shop like this. I used to buy a lot of things mainly because they were on sale and seemed like a great deal.
I now hear people say they needed to buy something because they had a coupon for a certain percentage off or that it was on sale. It may be a deal if it is something you have been looking for or something you need, but if you are buying something just because you have a coupon that is why the companies send out coupons and run sales. And in some cases, people may think since they saved money on their purchase, that means they have more money to spend on other items.
The same thing is true with shopping at dollar stores and thrift stores, things are inexpensive but those dollar items can add up quick. I am not saying shopping at these kinds or stores is a bad thing. I shop at them regularly, but like at other stores you need to think about what you are buying and if it something you really need or if you are just buying it because it seems like a good deal.
Years ago, the daughter of my boss at the time was looking at a sale ad and said she could save a certain percentage off. Her daughter, who was in grade school at the time told her "If you don't go shopping, you can save 100%".
Literally moments after I posted this, I ran across this post by Jean Chatzky about The Psychology of Bargains.