The new piece of Artemy Lebedev's .com/Mandership has an interesting observation about designer's mentality (thanks Dmitri for translation from Russian):

You can easily confuse the designer by asking: "What if the customer is smarter than you?"

His entire mentality is based on the belief that he is THE creator, and any idea he can possibly think of is impeccable by definition. The majority of designers the author has a pleasure to be acquainted with are under delusion that their craft has something to do with art. They also believe that their work is valuable simply because they did it.

A bottle, being an ideal object, is not shaped like this because its designer fancied narrow bottlenecks so much. It's rather because cork was such such a rare commodity.

Being a hobbyist UI designer, it seems like at a times I fall into that majority myself. Though in my defense, I do test things on myself and even use the UI I created on a daily basis. It is actually a hard work to convince other users that some of my creations are great. ;-)