For a designer of any kind, subjectivity is one of the hardest things to overcome when designing for other people. It is much easier to design what we want than to ask our customers/users what they want. When a design is subjective, it is often based on nothing more than our opinion, and we can easily mistake our opinion for that of our customers/users. When this happens, problems can occur.
Consider, for example, the mindset of a designer who has knowledge and expertise in a specific area. Perhaps, they prefer one type of interaction to another. Perhaps, that same interaction would seem confusing to someone else. What seems easy or second nature to one person can be totally incomprehensible to another. Think of a musician who has been playing the guitar for a number of years compared to someone who is just learning to play. The experienced musician cannot remember what it was like when they first learned to play all those years ago. What seems simple to them now would...