Because you are conditioned to think something is better and you don't trust your own senses.
An experiment in 2001 at the University of Bordeaux had wine experts taste a red and white wine, to determine which was the best. They dutifully explained what they liked about each wine but what they didn't realize was that scientists had just dyed the same white wine red and told them it was red wine.
The tasters described the sorts of berries and tannins they could detect in the red wine as if it really was red. Another test had them judge a cheap bottle of wine and an expensive one. They rated the expensive wine much more highly than the cheap, with much more flattering descriptions. It was actually the same wine.
It's not to say wine-tasting is pointless, it's to show that expectation can radically change experience. Yes, these people were experts, but that doesn't mean they can't be influenced by the same things as the rest of us, whether it be presentation or advertising or price.
This drives home the idea that reality is a construction of the brain. You don't passively receive the outside world, you actively construct your experience moment by moment.
And I think a lot of this generation is being plagued by some of the same influences.