Jakob Nielsen:

One of usability’s most hard-earned lessons is that you are not the user. This is why it’s a disaster to guess at the users’ needs. Since designers are so different from the majority of the target audience, it’s not just irrelevant what you like or what you think is easy to use — it’s often misleading to rely on such personal preferences.
 For sure, anybody who works on a design project will have a more accurate and detailed mental model of the user interface than an outsider. If you target a broad consumer audience, you will also have a higher IQ than your average user, higher literacy levels, and, most likely, you’ll be younger and experience less age-driven degradation of your abilities than many of your users.