It's called trust and it causes you to let your guard down.  

We had just started shopping at Walmart as we had a new family and bought a toy monster truck and our 2.5 year old dropped it from the grand total height of 13.5 inches and it broke.  Not fit for purpose.  We still shop at Walmart but will not buy anything but name brand products, and even then we suspect they do a lot of "special" Walmart runs.

Other stores only have profit in mind.  We avoid them like the plague - packaging - slicing and dicing - providing negative value.   Two large well known national brands have been bought out fairly recently as that model is doomed to failure.  

With Costco you know they have assessed the product and said, "Is this the best value for our customers?"  I do not know anyone else that does that.  

You then shop without having to consider the motives of the business - they have your best interests in mind - so you shop without the same scrutiny as you would other places.